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How to call functions in js. Function exit and its result. Function Overloading in JavaScript

Functions

A function is a block program code in JavaScript, which is defined once and can be executed, or called, repeatedly. You may already be familiar with the concept of a function under another name, such as a subroutine or procedure. Functions can have parameters: A function definition can include a list of identifiers, which are called parameters and act as local variables in the body of the function.

When calling functions, they can be passed values ​​or arguments corresponding to their parameters. Functions often use their arguments to calculate a return value, which is the value of the function call expression. In addition to the arguments, when any function is called, it is passed one more value that defines the context of the call - the value in the this keyword.

Functions in JavaScript are objects and can be used in different ways. For example, functions can be assigned to variables and passed to other functions. Because functions are objects, you can assign values ​​to their properties and even call their methods.

JavaScript allows function definitions to be nested within other functions, and such functions will have access to all variables present in the scope of the definition.

Defining Functions

A function definition begins with the function keyword, followed by the following components:

An identifier that specifies the name of the function

The name is a required part of the function declaration statement: it will be used to create a new variable to which the object will be assigned new feature. In function definition expressions, the name may be absent: if present, the name will refer to the function object only in the body of the function itself.

Pair parentheses around a list of zero or more identifiers, separated by commas

These identifiers will define the names of the function parameters and can be used as local variables in the function body.

A pair of curly braces with zero or more JavaScript instructions inside

These instructions make up the body of the function: they are executed every time the function is called.

The following example shows several function definitions in the form of statements and expressions. Note that function definitions as expressions are only useful if they are part of larger expressions, such as an assignment or function call, that do something with the newly declared function:

// Prints the names and values ​​of all properties of object obj function printprops(obj) ( for(var p in obj) console.log(p + ": " + obj[p] + "\n"); ) // Calculates the distance between points (x1,y1) and (x2,y2) function distance(x1, y1, x2, y2) ( var dx = x2 - x1; var dy = y2 - y1; return Math.sqrt(dx*dx + dy*dy ); ) // Recursive function(calling itself), calculating factorial function factorial(x) ( if (x

Note that in function definition expressions, the function name may not be present. The function declaration statement actually declares a variable and assigns it a function object.

A function definition expression, on the other hand, does not declare a variable. However, in definition expressions it is possible to specify the name of the function, as in the factorial function above, which may be required in the body of the function to call itself. If the function definition expression includes a name, given name will refer to a function object in the scope of that function. In effect, the function name becomes a local variable, accessible only in the function body. In most cases, the function name does not need to be specified in definition expressions, making definitions more compact.

Note that most (but not all) of the functions in the example contain a return statement. The return statement terminates the function and returns the value of its expression (if specified) to the calling program. If there is no expression in the return statement, it returns undefined. If there is no return statement in the function, the interpreter will simply execute all statements in the function body and return undefined to the calling program.

Most of the functions in the example evaluate to some value and use a return statement to return that value to the calling program. The printprops() function is slightly different in this sense: its job is to print the names of the properties of an object. It does not need to return any value, so there is no return statement in the function. The printprops() function will always return undefined. (Functions that do not have a return value are sometimes called procedures.)

Calling functions

The program code that forms the body of a function is executed not at the time the function is defined, but at the time it is called. Function calls are made using a call expression. A call expression consists of a function call expression that returns a function object, followed by parentheses with a comma-separated list of zero or more argument expressions inside.

If a function call expression is a property call expression - if the function is an object property or an array element (that is, a method) - then the call expression is a method call expression. The following snippet shows several examples of common function call expressions:

Printprops((x:4, age: 24)); var d = distance(1,1,5,6); var f = factorial(5) / factorial(12); f = square(5);

When a function is called, all argument expressions (specified between parentheses) are evaluated and the resulting values ​​are used as arguments to the function. These values ​​are assigned to parameters whose names are listed in the function definition. In the body of the function, parameter call expressions return the values ​​of the corresponding arguments.

When a normal function is called, the function's return value becomes the value of the calling expression. If a function returns after the interpreter has reached its end, undefined is returned. If a function returns as a result of a return statement, the value of the expression following the return statement is returned, or undefined if the return statement has no expression.

A method is nothing but a function that is stored as a property of an object. If you have a function func and an object obj, then you can define a method on obj called method as shown below:

// Define a simple object and function var obj = (); function func(a, b) ( return a+b;) // Add a method to object obj obj.method = func; // Now you can call this method var result = obj.method(4, 5);

Most often, when calling methods, the form of property access using the dot operator is used, but you can also use the form of property access using square brackets. For example, both of the following expressions are method call expressions:

Result = obj.method(4, 5); result = obj["method"](4, 5);

The arguments and return value of a method call are processed in the same way as a normal function call. However, the method call has one thing important difference: call context. A property access expression consists of two parts: an object (in in this case obj) and property name (method). In such method call expressions, obj becomes the calling context, and the function body is able to reference that object using the this keyword. For example:

Var obj = ( x: 0, y: 0, // Add method: function(a, b) ( this.x = a; this.y = b; ), // Another method sum: function() ( return this.x + this.y ) ); // Calling methods obj.add(15, 4); console.log(obj.sum()); // 19

Methods and the this keyword are central to the object-oriented programming paradigm. Any function used as a method actually receives an implicit argument - the object on which it was called. Typically, methods perform some operation on an object, and the method call syntax clearly reflects the fact that the function operates on the object.

Please note: this is a keyword, not a variable or property name. JavaScript Syntax does not allow assignment of values ​​to the this element.

Function Arguments and Parameters

In JavaScript, function definitions do not specify parameter types, and function calls do not perform any type checking on the argument values ​​passed. In fact, JavaScript doesn't even check the number of arguments when calling functions. The subsections below describe what happens if the number of arguments in a function call is less than or more number declared parameters. They also demonstrate how you can explicitly check the types of a function's arguments if you need to ensure that the function is not called with invalid arguments.

Optional Arguments

When the number of arguments in a function call is less than the number of declared parameters, the missing arguments are set to undefined. It is often convenient to write functions so that some of the arguments are optional and can be omitted when calling the function. In this case, it is desirable to provide the ability to assign reasonably reasonable default values ​​to parameters that may be omitted. For example:

// Add enumerable names of // properties of object obj to array arr and return it. If the argument // arr was not passed, create and return a new array function getPropertyNames(obj, /* optional */ arr) ( if (arr === undefined) arr = ; // If the array is not defined, create a new for( var property in obj) arr.push(property); return arr; // This function can be called with 1 or 2 arguments: var a = getPropertyNames((x:1, y:1)); // Get the properties of the object in a new array getPropertyNames((z:5),a); // add the properties of the new object to this array console.log(a); // ["x", "y", "z"]

Note that when declaring functions, optional arguments must complete the argument list in order to be omitted. The programmer who will write a call to your function will not be able to pass the second argument and at the same time omit the first: he will be forced to explicitly pass the value undefined in the first argument. Also note the /* optional */ comment in the function definition, which emphasizes the fact that the parameter is optional.

Variable length argument lists

If the number of arguments in a function call exceeds the number of parameter names, the function is unable to directly access unnamed values. The solution to this problem is provided by the Arguments object. In the function body, the arguments identifier refers to the Arguments object present in the call. The Arguments object is an array-like object that allows values ​​passed to a function to be retrieved by their numbers rather than their names.

Let's assume that a function func has been defined that requires one argument x. If you call this function with two arguments, the first one will be available inside the function by the name of the parameter x or as arguments. The second argument will only be available as arguments. Additionally, like real arrays, arguments has a length property that specifies the number of elements it contains. That is, in the body of a function called func with two arguments, arguments.length has the value 2.

The Arguments object can be used with the most different purposes. The following example shows how to use it to check if a function was called with the correct number of arguments, something JavaScript won't do for you:

Function func(x, y, z) ( // First checks to see if the correct number of arguments is passed if (arguments.length != 3) ( throw new Error("Func called with " + arguments.length + " arguments and required 3."); ) // And now the function code itself... )

Note that it is often not necessary to check the number of arguments, as in this example. The default behavior of the JavaScript interpreter is fine for most cases: missing arguments are replaced with the value undefined, and extra arguments are simply ignored.

The Arguments object illustrates an important feature of JavaScript functions: they can be written to take any number of arguments. The following function takes any number of arguments and returns the value of the largest one (the built-in Math.max() function behaves similarly):

Function maxNumber() ( var m = Number.NEGATIVE_INFINITY; // Loop through all arguments, find and // store the largest one for(var i = 0; i m) m = arguments[i]; // Return highest value return m; ) var largest = maxNumber(1, 10, 100, 2, 3, 1000, 4, 5, 10000, 6); // 10000

Functions like this that can take an arbitrary number of arguments are called variadic functions, variable arity functions, or varargs functions. This term arose with the advent of the C programming language.

Note that variadic functions must not be allowed to be called with an empty argument list. It makes perfect sense to use the arguments object when writing a function that expects a fixed number of required named arguments, followed by an arbitrary number of optional unnamed arguments.

Don't forget that arguments isn't actually an array - it's an Arguments object. Each Arguments object has numbered array elements and a length property, but is not technically an array. It's better to think of it as an object that has some numbered properties.

In addition to its array elements, the Arguments object defines the callee and caller properties. Trying to change the values ​​of these properties in ECMAScript 5 strict mode is guaranteed to throw a TypeError exception. However, in lax mode, the ECMAScript standard states that the callee property refers to the executable in this moment function. The caller property is not standard, but it is present in many implementations and refers to the function that called the current one.

The caller property can be used to access the call stack, and the callee property is especially useful for recursively calling unnamed functions:

Var factorial = function(x) ( if (x

Function Properties and Methods

We have seen that functions can be used as values ​​in JavaScript programs. The typeof operator returns the string "function" for functions, but functions are actually a special kind of object in JavaScript. And since functions are objects, they have properties and methods like any other objects. There is even a Function() constructor that creates new function objects. The following subsections describe the properties and methods of functions.

length property

In the function body, the arguments.length property specifies the number of arguments passed to the function. However, the length property of the function itself has a different meaning. This read-only property returns the number of arguments the function expects to receive - the number of declared parameters.

The following snippet defines a function called check() that receives an array of arguments from another function. It compares the arguments.length property (the number of arguments actually passed) with the arguments.callee.length property (the number of expected arguments) to determine whether the function has been passed as many arguments as it expects. If the values ​​do not match, an exception is thrown. The check() function is followed by a test function, func(), that demonstrates how to use the check() function:

// This function uses arguments.callee, so it // will not work in strict mode function check(args) ( var actual = args.length; // Actual number of arguments var expected = args.callee.length; // Expected number arguments if (actual !== expected) // If they do not match, an exception is thrown throw new Error("expected: " + expected + "; received " + actual ) function func(x, y, z) ( // Check number of expected and actually passed arguments check(arguments); // Now execute the rest of the function return x + y + z )

prototype property

Every function has a prototype property, which refers to an object known as the prototype object. Each function has its own prototype object. When a function is used as a constructor, the newly created object inherits the properties of that prototype object.

Prototypes and the prototype property were discussed in a previous article.

call() and apply() methods

The call() and apply() methods allow you to call a function indirectly, as if it were a method on some other object. The first argument to both call() and apply() methods is the object on which the function is called; this argument specifies the context of the call and becomes the value of the this keyword in the function body. To call func() (without arguments) as a method of obj, you can use any of the call() or apply() methods:

Func.call(obj); func.apply(obj);

Either way of calling it is equivalent to the following snippet (assuming obj doesn't have a property named m):

Obj.m = func; // Temporarily make func a method obj obj.m(); // Call it without arguments. deleteobj.m; // Remove temporary method.

In ECMAScript 5 strict mode, the first argument of the call() and apply() methods becomes the value of this, even if it is a simple value, null, or undefined. In ECMAScript 3 and in lax mode, the values ​​null and undefined are replaced by the global object, and the simple value is replaced by the corresponding wrapper object.

All other arguments to the call() method following the first argument specifying the calling context are passed to the called function. The apply() method acts like the call() method, except that the arguments to the function are passed as an array. If a function is capable of processing an arbitrary number of arguments, the apply() method can be used to call such a function in the context of an array of arbitrary length.

The following example demonstrates practical use call() method:

// Below are two functions that display the properties and // property values ​​of an arbitrary object. Method // displays are passed as an argument func function print1(func, obj) ( for (n in obj) func(n +": " + obj[n]); ) function print2(func, objDevice, obj) ( for ( n in obj) func.call(objDevice, n +": " + obj[n] ) var obj = (x:5, y:10); print2(document.write, document, obj); // Works correctly print2(console.log, console, obj); print1(document.write, obj); // An Illegal invocation exception will occur because print1(console.log, obj); // it is impossible to call these methods without a context object

bind() method

The bind() method first appeared in ECMAScript 5, but is easy to imitate in ECMAScript 3. As its name suggests, the main purpose of the bind() method is to bind a function to an object. If you call func's bind() method and pass it an obj object, it will return a new function. Calling a new function (as a normal function) will make the call original function func as a method of object obj. Any arguments passed to the new function will be passed to the original function. For example:

// Function to bind function func(y) ( return this.x + y; ) var obj = (x:1); // Object to bind to var g = func.bind(obj); // Calling g(x) will call obj.func(x)

This type of binding is easy to implement in ECMAScript 3, as shown below:

// Returns a function that calls func as a method of object obj // and passes it all its arguments function bind(func, obj) ( if (func.bind) return func.bind(obj); // Use bind method if available else return function() ( // Otherwise bind as below return func.apply(obj, arguments); )

The bind() method in ECMAScript 5 does more than just bind a function to an object. It also performs partial casting: in addition to the this value, all arguments passed to the bind() method after its first argument will be bound. Partial application is a common technique in functional programming and is sometimes called currying.

Let's start with the fact that the JavaScript language supports the concept of OOP (object oriented programming). This concept is that there are such elements as objects and these objects have various properties and methods (functions) that allow you to manipulate them.

A function is a separate block of code that consists of one or more operators. It has its own (unique) name and can take various parameters, depending on which it can perform one or another operation.

A method is also a function, but it already belongs to some class or object.

In order to call a method, you must first write the name of the object, then write the name of the method separated by a dot. The exception to this rule is when calling the alert(), confirm(), and prompt() methods of the window object. They can be called without specifying the object name. We have already become acquainted with these methods in this article.

Also, in previous articles we were introduced to the document.write() output method, which belongs to the document object.

So, in programming there is a very important opportunity, which is that you can create your own functions.

The function syntax looks like this:


For example, let's create simple function, which will add the passed text to the paragraph and output it. And it will also make it bold and italic.

Function writeText(text)( //Add text in the paragraph and display it document.write("

"+text+"

"); ) //Call the created function writeText("Hello!");

Save the document and open it in the browser.


Comment! When declaring a function, curly braces must be present, no matter how many operators there are.

Why are functions needed in programming?

The main advantage of using the function is the reduction in size source code script.

Let's say we need to iterate over three one-dimensional arrays. As we know from this article: , the array is iterated using a loop. Without the function, the code for this script will look like this:

//declare three arrays var arr1 = ; var arr2 = ["b", 5, 9.2, "h", 8, 2]; var arr2 = ; for(var i = 0; i< arr1.length; i++){ document.write("

The array element arr1, with index " + i + " is equal to: "+ arr1[i] +"

"); ) for(var i = 0; i< arr2.length; i++){ document.write("

The array element arr2, with index " + i + " is equal to: "+ arr2[i] +"

"); ) for(var i = 0; i< arr3.length; i++){ document.write("

The array element arr3, with index " + i + " is equal to: "+ arr3[i] +"

"); }

So, in order not to write your own loop for each array, it is better to use a function in which we pass the array, and it will display all its elements on the screen. Thus, we, firstly, reduce the size of the code, and secondly, get rid of repeating code.

Function printArr(arr)( for(var i = 0; i< arr.length; i++){ document.write("

The array element with index " + i + " is equal to: "+ arr[i] +"

"); ) ) //declare three arrays var arr1 = ; var arr2 = ["b", 5, 9.2, "h", 8, 2]; var arr2 = ; //Call the created function to iterate through each array printArr (arr1); printArr(arr2); printArr(arr3);

Function parameters

A function can take any number of parameters, from one to infinity. Or, it can be completely without parameters.

Let's create a parameterless function that will simply print the classic "Hello world" phrase to the screen.

Function helloWorld())( document.write("Hello World"); ) //Call the function without parameters, helloWorld helloWorld();

Any function parameter can have its own default value. This means that if we do not pass any value when calling the function this parameter, then it uses its default value.

For example, let's create a function that adds two passed numbers. If we pass only one number, then, by default, the second number will be 4.

Function summa(number1, number2 = 4)( document.write("

The sum of the numbers " + number1 + "(First parameter) and " + number2 + "(Second parameter) is equal to: " + (number1 + number2) + "

"); ) //Call a function that, by default, will output the result of adding the passed number, with the number 4. summa(5); // Result: 9 //If we also supply the second parameter, the function will output the result of adding the numbers from both parameters .summa(5, 20); // Result: 25

It is also possible that inside a function one could call another existing function.

For example, let's call the first function we created, writeText(), inside the previous summa() function. We will pass the result of adding numbers to the writeText() function. In this case, the code for the summa() function will look like this:

Function summa(number1, number2 = 4)( writeText(number1 + number2); ) //Call the function summa summa(5); // Result: 9 summa(5, 20); // Result: 25

Functions that return some value

So far we have written functions that display the result on the screen immediately.

Now let's learn how to write a function that returns some result. We can add this result to some variable and work with it further.

In order to better understand what we are talking about, let’s remember methods such as prompt() and confirm(). These methods actually return the value received from the user, rather than displaying it.

For example, let's create our own function that will return the last element of the array passed as a parameter.

Function lastElement(arr)( //Return the last element of the passed array return arr; ) //Declare the array var otherArr = ["iphone", "asus", 2000, 9.8, "twix"]; //Call the created lastElement function and pass it the created array as a parameter otherArr var lastEl = lastElement(otherArr); //Display the resulting last element of the array alert(lastEl);

As a result, we will get the word 'twix', since this word is the last element of the otherArr array.

The alert() method does not return anything. That is, if we try to display a variable whose type contains the result of calling the alert() method, we will see the value undefined . This is the same as trying to display the value of an empty variable.

For example, let's take the result last call alert() from the previous example, put it in the resAlert variable and use the writeText function we created to try to display the result.

//Display the received last element of the array var resAlert = alert(lastEl); var test; writeText(resAlert); //undefined writeText(test); //undefined

As you can see, in both cases we received the value undefined.

Global and local variables

Global variables are those variables that are declared outside the function. That is, all those variables that are not declared inside the function itself are global. They are visible (valid) throughout the document.

Local variables are those variables that are declared within the function itself. And they are only valid within a given function. Outside of it, local variables will no longer work.

Local and global variables are not related to each other in any way.


In the example from the image, if we tried to print the contents of variable x, we would get an undefined message because we forgot to call the other() function.

Therefore, in order for the changes made inside the function to work, it is necessary to call this function.

We call the other() function, and if we now try to display the value of the variable x, we will see the number 4 as a result.

To access a global variable from inside a function, you don’t need to do anything, you just need to use it. Changes made to global variables will be visible outside the function.

Var x = 8; function increment() ( x++; ) //Call the function increment() increment(); alert(x); //Result: 9

If we do not want the global variable to change, we must declare a local variable (possibly with the same name as the global one) and all actions will be performed on it.

Var g = 100; function func())( var g = 14; g *= 2; // This is the same as g = g * 2 alert(g); // Result: 28 ) // Call the function. func(); alert(g);//Result: 100

That's all, dear readers, now you know what a function is, how to create your own function, how to call a function and what types of functions exist. You also learned what global and local variables are.

As I wrote at the beginning of the article, functions are very important elements, so you should know them perfectly.

Tasks
  • Create a function that takes two numbers as parameters and returns the result of multiplying those numbers.
  • Print the result.

    • Article under development!

    An article in which we will consider what a function is, as well as the traditional (classic) version of working with it. In addition, we will analyze what the arguments (parameters) of the function and the return operator are.

    What is a function?

    A function is a set of instructions that can be given a name and then accessed by that name from anywhere in the program.

    A classic example of using a function. The web page has JavaScript code, some fragment in which is repeated several times. To avoid this, you can format this fragment as a function, and then call it in the necessary places in the code using the name of this function. Calling this function will mean executing the instructions contained in it.

    How to organize the execution of some task in JavaScript using functions? To do this you usually do this:

    • break the task into its component parts (subtasks);
    • subtasks are formalized through functions;
    • develop the main code using calls to the created functions.

    As a result, such a program becomes more structured. It is easier to make various changes and add new features.

    Declaring and calling a function

    Operations with a function in JavaScript can be divided into 2 steps:

    • declaration (creation) of a function.
    • call (execute) this function.

    Function declaration. Creating a function in JavaScript begins with writing the keyword function, then the name of the function, then parameters are listed in parentheses x, if necessary, followed by instructions, which are enclosed in curly braces.

    // function declaration someName function someName() ( alert("You called function someName!"); ) JavaScript - Function declaration syntax

    Functions of this kind in JavaScript are called function declaration statements. In addition to this type, JavaScript also distinguishes between the function definition expression and arrow function expression functions.

    The function name is composed according to the same rules as the variable name. Those. it can contain letters, numbers (0-9), "$" and "_" signs. It is recommended to use only letters of the English alphabet (a-z, A-Z) as letters. The name of a function, just like the name of a variable, cannot begin with a number.

    A function can have as many parameters as desired or none at all. Parentheses are included in any case. If there are several parameters, then they must be separated by a comma. Function parameters are accessed by their name.

    A set of instructions enclosed in curly braces is the function code that will be executed when it is called.

    Function call. The declared function itself will not be executed. In order to run it, it must be called. A function is called by specifying its name and two parentheses. Arguments are specified inside parentheses if necessary.

    // call the function given in the previous example someName(); JavaScript - Function Call Syntax

    Is a function an object in JavaScript?

    Functions in JavaScript are objects. In JavaScript, everything is an object except the six primitive data types. And if the function is an object, then a reference to it can be stored in a variable.

    // function declaration someName function someName() ( alert("You called the someName function!"); ) var reference = someName;

    After this, you can call the function like this:

    Reference();

    Function parameters and arguments

    Function arguments are values ​​that are passed to a function when it is called. Arguments are separated from each other using a comma.

    // call the sayWelcome function passing two arguments to it sayWelcome("Ivan", "Ivanov"); // another call to the sayWelcome function with two arguments sayWelcome("Petr", "Petrov");

    Function parameters are one of the ways in JavaScript that you can access arguments within a function. The parameters of the function at the stage of its declaration are described in parentheses.

    In other words, function parameters are local variables that are created automatically when the function is launched. The parameters receive as values ​​the corresponding arguments passed to the function during its call. You can access the parameters only inside this function; outside of it they do not exist.

    // declaration of a function sayWelcome, which has two parameters function sayWelcome (userFirstName, userLastName) ( // an instruction that displays the values ​​of the parameters “userFirstName” and “userLastName” to the console console.log("Welcome, " + userLastName + " " + userFirstName ; )

    In JavaScript, when calling a function, the number of arguments does not have to be the same as the number of parameters. Parameters that were not set to value when called will be equal to undefined .

    For example, let's call the function from the example above, without specifying one or two parameters:

    // calling the sayWelcome function and passing one argument to it sayWelcome("Peter"); // Welcome, undefined Peter // calling the sayWelcome function without passing any arguments to it sayWelcome(); // Welcome, undefined undefined

    An example of a function that will simply output the arguments passed to it to the browser console:

    // function declaration function outputParam(param1, param2, param3) ( console.log(param1 + "; " + param2 + "; " + param3); ) // calls to the function outputParam passing it different quantities parameters outputParam("Rain","Snow","Fog"); // Rain; Snow; Fog outputParam(17); // 17; undefined; undefined outputParam(24,33); // 24; 33; undefined outputParam(); // undefined; undefined; undefined

    Another way to access arguments within a function is to use the special arguments object. Arguments are accessed via arguments in the same way as elements of a regular array, i.e. by their serial numbers. Thus, argument - will allow you to get the first argument, arguments - the second argument, etc.

    // function declaration sum function sum(num1, num2) ( /* num1 or arguments – get the value of argument 1 num2 or arguments – get the value of argument 2 */ var sum1 = num1 + num2, sum2 = arguments + arguments; return "Sum, obtained by the 1st method is equal to " + sum1 + "; the sum obtained by the 2nd method is " + sum2; ) /* output the result of the sum function to the console 7 - the first argument (it can be accessed either by the name num1 or using arguments) 4 - second argument (it can be accessed either by name num2 or using arguments) */ console.log(sum(7,4));

    The main difference between these methods is that the first of them allows you to access only those arguments that were given names at the function declaration stage. The second method allows you to get the value of any argument, even if it does not have a name (by serial number). This is an opportunity JavaScript language allows you to create universal flexible functions.

    In addition to receiving arguments, the arguments object also lets you know the number of arguments. This is done using the length property.

    You can iterate over the arguments passed to the function, for example, using for loop or for...of .

    // function declaration sum function sum() ( var i = 0; console.log("Output all arguments using a for loop"); for (i; i< arguments.length; i++) { console.log(i + 1 + " аргумент равен " + arguments[i]); } console.log("Вывод всех аргументов с помощью цикла for...of"); for (arg of arguments) { console.log(arg); } } // вызов функции sum sum(7, 4, 3, 1);

    A function that displays to the console all the arguments passed to it and their number:

    // function declaration function myFunction () ( var i; console.log("Number of parameters passed = " + arguments.length); // let's go through all the parameters using a for loop for (i = 0; i< arguments.length; i++) { console.log(i + " параметр = " + arguments[i]); } } // вызовы функции myFunction myFunction(3, 7, 27, "JavaScript"); myFunction(); myFunction("Яблоки", "Груши", "Апельсины");

    A function that performs the addition of all arguments passed to it (their number is unknown in advance):

    // function declaration var myCalc = function() ( // let's go through all the parameters using the for loop var i, sum = 0; for (i = 0; i lt; arguments.length; i++) ( sum += arguments[i] ; ) // return the sum as the result return sum; ) // function call (output to the console) console.log(myCalc(4, 20, 17, -6));

    As a result, using the arguments object you can implement the following in the function body:

    • checking the number of arguments passed;
    • processing any number of parameters.

    In addition to the function itself, other functions located in it also have access to the arguments that are passed to it at the call stage.

    Function mainF(p1, p2) ( function childF() ( console.log("p1 = " + p1 + "; p2 = " + p2); ) childF(); ) mainF(3, 5); // p1 = 3; p2 = 5 mainF(4, 7); // p1 = 4; p2 = 7

    Default Settings

    Starting with ECMAScript 2015 (6), a function parameter can be set to its default value.

    For example, let's set the "color" parameter to its default value, equal to "#009688":

    Function setBGColor(color = "#009688") ( document.body.style.backgroundColor = color; ) setBGColor(); // background color will be #009688 setBGColor("red"); // background color will be red

    Before ECMAScript 2015, you could set the parameter to its default value, for example, like this:

    Function setBGColor(color) ( color = color !== undefined ? color: "#009688"; // set color to the default value of "#009688" document.body.style.backgroundColor = color; )

    Remaining parameters

    If, when calling a function, you pass more arguments to it than it has parameters, then you can get the remaining ones using the so-called remaining parameters (rest patameters). This opportunity appeared in the language starting with ECMAScript 2015.

    // ...nums - the remaining parameters, which can be accessed in this case by name nums function doMath(mathAction, ...nums) ( var result = 0; nums.forEach(function(value) ( ​​switch (mathAction) ( case "sum": result += value; break; result = 0; ) return result; ) console.log(doMath("sum", 3, 4, 21, -4)); // 24 (3 + 4 + 21 + (-4)) console.log(doMath("sumSquare", 1, 4)); // 17 (1^2 + 4^2) console.log(doMath("sumCube", 3, 2, 4)); // 99 (3^3 + 2^3 + 4^3)

    return statement

    The return statement is intended to return a value or the result of evaluating an expression current function. The value or expression must be separated from the return by a space. In addition, the return statement stops the execution of the function, i.e. all instructions following it will not be executed.

    A function in JavaScript always returns a result, regardless of whether a return statement is used or not.

    // function that returns the result function sayWelcome (userFirstName, userLastName) ( if ((!userFirstName) || (!userLastName)) return "Welcome, anonymous user"; else return "Welcome, " + userLastName + " " + userFirstName ; ) // variable declaration person var person; // assign the result of the sayWelcome function to the person variable person = sayWelcome("Ivan","Ivanov"); // print the value of the variable to the console console.log(person); //Instruction that will output to the console the result of the sayWelcome function console.log(sayWelcome("Petr","Petrov")); //Instruction that will output to the console the result of the sayWelcome function console.log(sayWelcome("Sidorov")); JavaScript - Function with Parameter Checking

    A function in JavaScript always returns a result as a result of its execution, even if it is not explicitly defined using the return statement. This result is undefined.

    // 1. function that does not return any result function sayWelcome (userFirstName, userLastName) ( console.log("Welcome, " + userLastName + " " + userFirstName); ) // let's try to get the result from a function that does not return anything console .log(sayWelcome("Ivan", "Ivanov")); // 2. function containing a return statement without a value function sayDay (day) ( day = "Today," + day; return; //this instruction will not be executed because it comes after the return statement console.log(day) ; ) // let's try to get the result from a function that contains a return statement without a value console.log(sayDay("February 21, 2016")); JavaScript - Get value from a function that returns nothing

    The same result will be obtained if the return statement does not specify a return value.

    Function Overloading in JavaScript

    Function overloading in programming is the ability to declare multiple functions with same names in one scope. Such functions differ from each other in the type and number of arguments. Each function has its own program logic. Function overloading is used so that similar actions can be performed using a single function name.

    The JavaScript language does not support function overloading in the same way as it is implemented, for example, in C similar languages. Those. In JavaScript, you cannot create multiple functions with the same names that are in the same scope.

    Similar functionality can be implemented in JavaScript using the following steps:

    • To check whether an argument is passed or not, use a condition that checks its value for undefined .
    • To check the number of arguments passed to a function, use the arguments length property of the object.
    • To find out the type of the argument value passed, use the typeof or instanceof operators.
    • To work with a variable number of arguments, use the arguments object.
    • Starting with ECMAScript6, you can specify default values ​​for arguments.

    For example, let's create a function that can be called with one or two arguments:

    //declaration of a function that changes color background elements function setBgColor(bgColor,elements) ( //if the elements parameter is not specified when calling if (elements=== undefined) ( //then set its value to "div" elements = "div"; ) //get all elements elements = $(elements); // iterate through all elements and set them to the specified background color elements.each(function())( $(this).css("background-color",bgColor); )) /*Call the setBgColor function , specifying one parameter. Because 2 parameter is not specified, then this function will change the background color of all div elements.*/ setBgColor("green"); /*Call the setBgColor function, specifying 2 parameters. Because 2 parameter is specified, then this function will change the background color of the button elements only.*/ setBgColor("#ff0000","button");

    Let's make some changes to the above code. Namely, we specify the default value for the second parameter:

    //declaration of a function that changes the background color of elements //the elements parameter has the value "div" by default function setBgColor(bgColor,elements = "div") ( //get all elements elements = $(elements); //iterate all elements and set them to the specified background color elements.each(function())( $(this).css("background-color",bgColor); ) ) //call the setBgColor function, specifying one parameter setBgColor("green" ); //call the setBgColor function, specifying 2 parameters setBgColor("#ff0000","button");

    An example of how in JavaScript you can implement an “overloaded” function that calculates the number of calories a person needs per day:

    // description of the function function countCal(sex, height) ( // parameters: sex (sex) and height (height) var result; if ((sex === 0) || (sex === "man")) ( result = (height - 100) * 20; ) else if ((sex === 1) || (sex === "woman")) ( result = (height - 105) * 19; ) if (result) ( // arguments - activity level if (arguments) ( result *= arguments; ) console.log("Amount of kcal for normal life: " + result) else ( console.log("Incorrect parameters"); ) ) / * calling a function and passing 2 arguments to it (1 - "man", it can be accessed using the name sex and arguments; 2 - the value 185, it can be accessed using the name sex and arguments) */ countCal("man", 185); /* calling a function and passing 3 parameters to it, although only 2 are present in the function description (in this case, you can get the value of the 3rd parameter only as arguments) */ countCal(0, 185, 2);

    Recursion

    Recursion is a call within the body of some function to itself.

    A function is usually called depending on how it is declared by name or by a variable containing a reference to the function.

    Function fact(n) ( if (n === 1) ( return 1; ) return fact(n-1) * n; ) console.log(fact(5)); // 120

    You can call a function inside its body not only by name, but also by using the callee property of the arguments object. But this property It's better not to use it, because... it is outdated. In addition, in strict mode it does not work at all.

    What are built-in (standard) functions?

    JavaScript has a huge set of built-in (standard) functions. These functions are already described in the browser engine itself. Almost all of them are methods of one object or another.

    For example, in order to call the built-in function (method) alert, it does not need to be declared first. It is already described in the browser. The alert method is called by specifying a name, parentheses, and an argument inside them. This method designed to display a message on the screen in the form of a dialog box. Text message is taken from the parameter value of this function.

    // calling the alert function alert("Some text"); JavaScript - Calling the alert function

    9 answers

    There are several ways to handle events using HTML/DOM. There's no real right or wrong way, but different ways useful in different situations.

    1: Defines it in HTML:

    2: Add it to the DOM property of the event in Javascript:

    //- Using a function pointer: document.getElementById("clickMe").onclick = doFunction; //- Using an anonymous function: document.getElementById("clickMe").onclick = function () ( alert("hello!"); );

    3: And adds a function to the event handler using Javascript:

    Var el = document.getElementById("clickMe"); if (el.addEventListener) el.addEventListener("click", doFunction, false); else if (el.attachEvent) el.attachEvent("onclick", doFunction);

    Both the second and third methods allow inline/anonymous functions, and both must be declared after the element has been parsed from the document. The first method is not valid XHTML because the onclick attribute is not specified in the XHTML specification.

    The first and second methods are mutually exclusive, that is, using one (the second) will override the other (the 1st). The third method will allow you to connect as many functions as you like to the same event handler, even if the first or second method was used.

    Most likely the problem is somewhere in your CapacityChart() function. After visiting your link and running your script, the CapacityChart() function runs and two popups open (one of them is closed according to the script). If you have the following line:

    CapacityWindow.document.write(s);

    Try this instead:

    CapacityWindow.document.open("text/html"); CapacityWindow.document.write(s); CapacityWindow.document.close();

    CHANGE
    When I saw your code, I thought you were writing it specifically for IE. As mentioned, you will need to replace the references to document.all with document.getElementById . However, you will still have to fix the script after this, so I would recommend having it work in IE at least, as any mistakes you make by changing the code to work cross-browser could cause even more confusion. Once it works in IE, it will be easier to see if it works in other browsers while you update the code.

    I'd say it would be better to add javascript in an unobtrusive manner...

    if you are using jQuery you can do something like:

    $(document).ready(function())( $("#MyButton").click(function())( CapacityChart(); )); ));

    Your HTML and the way you are calling the function from the button looks correct.

    The problem is in the CapacityCount function. I'm getting this error in my console on Firefox 3.5: "document.all is undefined" on line 759 of bendelcorp.js.

    It looks like document.all is IE only and is in a non-standard way DOM access. If you use document.getElementById() this should work. Example: document.getElementById("RUnits").value instead of document.all.Capacity.RUnits.value

    One of the main problems you run into is when you use browser sniffing without a good reason:

    If(navigator.appName == "Netscape") ( vesdiameter = document.forms["Volume"].elements["VesDiameter"].value; // more stuff snipped ) else ( vesdiameter = eval(document.all.Volume. VesDiameter.value); // more stuff snipped )

    I'm on Chrome, so navigator.appName won't be Netscape . Does Chrome support document.all ? Maybe, but then again, maybe not. What about other browsers?

    The version of the code in the Netscape fork should work on any browser back to Netscape Navigator 2 since 1996, so you should probably just stick with that... except that it won't work (or isn't guaranteed to work) because you didn't specify name attribute for input elements, so they won't be added to the form's elements array as named elements:

    Either give them a name and use an elements array, or (better) use

    Var vesdiameter = document.getElementById("VesDiameter").value;

    which will work for everyone modern browsers- no branching required. To be on the safe side, replace this by sniffing the browser version greater than or equal to 4, with a check for getElementById support:

    If (document.getElementById) ( // NB: no brackets; we"re testing for existence of the method, not executing it // do stuff... )

    You'll probably also want to validate your input; something like

    Var vesdiameter = parseFloat(document.getElementById("VesDiameter").value); if (isNaN(vesdiameter)) ( alert("Diameter should be numeric"); return; )

    Last update: 04/09/2018

    Functions are a set of instructions that perform specific action or calculating a specific value.

    Function definition syntax:

    Function function_name([parameter [, ...]])( // Instructions )

    A function definition begins with the keyword function followed by the name of the function. A function name follows the same rules as a variable name: it can only contain numbers, letters, underscores, and dollar ($) characters, and must begin with a letter, underscore, or dollar.

    After the function name, the parameters are listed in parentheses. Even if a function has no parameters, it simply contains empty parentheses. Then, in curly braces, comes the body of the function, which contains a set of instructions.

    Let's define the simplest function:

    Function display())( document.write("function in JavaScript"); )

    This function is called display(). It doesn't accept any parameters and all it does is write a string to the web page.

    However simple definition The feature is not yet sufficient for it to work. You still need to call her:

    function display())( document.write("function in JavaScript"); ) display();

    It is not necessary to give functions a specific name. You can use anonymous functions:

    Var display = function())( // function definition document.write("function in JavaScript"); ) display();

    What we're doing is actually defining a display variable and assigning it a function reference. And then by name variable function called.

    We can also dynamically assign functions to a variable:

    Function goodMorning())( document.write("Good morning"); ) function goodEvening())( document.write("Good evening"); ) var message = goodMorning; message(); // Good morning message = goodEvening; message(); // Good evening

    Function parameters

    Let's consider passing parameters:

    Function display(x)( // function definition var z = x * x; document.write(x + " squared equals " + z); ) display(5); // function call

    The display function takes one parameter - x. Therefore, when calling a function, we can pass a value for it, for example, the number 5, as in this case.

    If a function takes several parameters, then using the spread operator... we can pass a set of values ​​for these parameters from an array:

    Function sum(a, b, c)( let d = a + b + c; console.log(d); ) sum(1, 2, 3); let numbers = ; sum(...nums);

    In the second case, numbers from the nums array are passed to the function. But in order to transmit not just an array as one value, but numbers from this array, the spread operator (ellipsis...) is used.

    Optional Parameters

    A function can take many parameters, but some or all of the parameters can be optional. If no value is passed for parameters, they default to "undefined".

    Function display(x, y)( if(y === undefined) y = 5; if(x === undefined) x = 8; let z = x * y; console.log(z); ) display() ; // 40 display(6); // 30 display(6, 4) // 24

    Here the display function takes two parameters. When calling a function, we can check their values. However, when calling a function, it is not necessary to pass values ​​for these parameters. To check for the presence of parameter values, a comparison with the undefined value is used.

    There is another way to define default values ​​for parameters:

    Function display(x = 5, y = 10)( let z = x * y; console.log(z); ) display(); // 50 display(6); // 60 display(6, 4) // 24

    If no values ​​are passed to the parameters x and y, they are obtained as the values ​​of the numbers 5 and 10, respectively. This method is more concise and intuitive than comparing with undefined.

    In this case, the default value of the parameter can be derived, representing the expression:

    Function display(x = 5, y = 10 + x)( let z = x * y; console.log(z); ) display(); // 75 display(6); // 96 display(6, 4) // 24

    In this case, the value of the parameter y depends on the value of x.

    If necessary, we can get all the passed parameters through the globally accessible arguments array:

    Function display())( var z = 1; for(var i=0; i