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    Functions and function scope

    This translation is in progress.

    Generally speaking, a function is a "subprogram" that can be called by code external (or internal in the case of recursion) to the function. Like the program itself, a function is composed of a sequence of statements called the function body. Values can be passed to a function, and the function can return a value.

    In JavaScript, functions are first-class objects, i.e. they are objects and can be manipulated and passed around just like any other object. Specifically, they are Function objects.

    For more examples and explanations, see also the JavaScript guide about functions.

    Description

    Every function in JavaScript is a Function object. See Function for information on properties and methods of Function objects.

    Functions are not the same as procedures. A function always returns a value, but a procedure may or may not return any value.

    To return a specific value other than the default, a function must have a return statement that specifies the value to return. A function without a return statement will return a default value. In the case of a constructor called with the new keyword, the default value is the value of its this parameter. For all other functions, the default return value is undefined.

    The parameters of a function call are the function's arguments. Arguments are passed to functions by value. If the function changes the value of an argument, this change is not reflected globally or in the calling function. However, object references are values, too, and they are special: if the function changes the referred object's properties, that change is visible outside the function, as shown in the following example:

    /* Declare the function 'myFunc' */
    function myFunc(theObject) {
       theObject.brand = "Toyota";
     }
     
     /*
      * Declare variable 'mycar';
      * create and initialize a new Object;
      * assign reference to it to 'mycar'
      */
     var mycar = {
       brand: "Honda",
       model: "Accord",
       year: 1998
     };
    
     /* Logs 'Honda' */
     console.log(mycar.brand);
    
     /* Pass object reference to the function */
     myFunc(mycar);
    
     /*
      * Logs 'Toyota' as the value of the 'brand' property
      * of the object, as changed to by the function.
      */
     console.log(mycar.brand);
    

    The this keyword does not refer to the currently executing function, so you must refer to Function objects by name, even within the function body.

    Defining functions

    There are several ways to define functions:

    The function declaration (function statement)

    There is a special syntax for declaring functions (see function statement for details):

    function name([param[, param[, ... param]]]) {
       statements
    }
    
    name
    The function name.
    param
    The name of an argument to be passed to the function. A function can have up to 255 arguments.
    statements
    The statements comprising the body of the function.

    The function expression (function expression)

    A function expression is similar to and has the same syntax as a function declaration (see function expression for details):

    function [name]([param] [, param] [..., param]) {
       statements
    }
    
    name
    The function name. Can be omitted, in which case the function becomes known as an anonymous function.
    param
    The name of an argument to be passed to the function. A function can have up to 255 arguments.
    statements
    The statements which comprise the body of the function.

    The generator function declaration (function* statement)

    Note: Generator function are an experimental technology, part of the ECMAScript 6 proposal, and are not widely supported by browsers yet.

    There is a special syntax for declaration generator functions (see function* statement for details):

    function* name([param[, param[, ... param]]]) {
       statements
    }
    
    name
    The function name.
    param
    The name of an argument to be passed to the function. A function can have up to 255 arguments.
    statements
    The statements comprising the body of the function.

    The generator function expression (function* expression)

    Note: Generator function are an experimental technology, part of the ECMAScript 6 proposal, and are not widely supported by browsers yet.

    A generator function expression is similar to and has the same syntax as a generator function declaration (see function* expression for details):

    function* [name]([param] [, param] [..., param]) {
       statements
    }
    
    name
    The function name. Can be omitted, in which case the function becomes known as an anonymous function.
    param
    The name of an argument to be passed to the function. A function can have up to 255 arguments.
    statements
    The statements which comprise the body of the function.

    The arrow function expression (=>)

    Note: Arrow function expressions are an experimental technology, part of the ECMAScript 6 proposal, and are not widely supported by browsers yet.

    An arrow function expression has a shorter syntax and lexically binds its this value (see arrow functions for details):

    ([param] [, param]) => {
       statements
    }
    
    param => expression
    
    param
    The name of an argument. Zero arguments need to be indicated with (). For only one argument the parentheses are not required. (like foo => 1)
    statements or expression
    Multiple statements need to be enclosed in brackets. A single expression requires no brackets. The expression is also the implicit return value of that function.

    The Function constructor

    Note: Using the Function constructor to create functions is not recommended since it needs the function body as a string which may prevent some JS engine optimizations and can also cause other problems.

    As all other objects, Function objects can be created using the new operator:

    new Function (arg1, arg2, ... argN, functionBody)
    
    arg1, arg2, ... argN
    Zero or more names to be used by the function as formal argument names. Each must be a string that conforms to the rules for a valid JavaScript identifier or a list of such strings separated with a comma; for example "x", "theValue", or "a,b".
    functionBody
    A string containing the JavaScript statements comprising the function definition.

    Invoking the Function constructor as a function (without using the new operator) has the same effect as invoking it as a constructor.

    The GeneratorFunction constructor

    Note: Arrow function expressions are an experimental technology, part of the ECMAScript 6 proposal, and are not widely supported by browsers yet.

    Note: GeneratorFunction is not a global object, but could be obtained from generator function instance (see GeneratorFunction for more detail).

    Note: Using the GeneratorFunction constructor to create functions is not recommended since it needs the function body as a string which may prevent some JS engine optimizations and can also cause other problems.

    As all other objects, GeneratorFunction objects can be created using the new operator:

    new GeneratorFunction (arg1, arg2, ... argN, functionBody)
    
    arg1, arg2, ... argN
    Zero or more names to be used by the function as formal argument names. Each must be a string that conforms to the rules for a valid JavaScript identifier or a list of such strings separated with a comma; for example "x", "theValue", or "a,b".
    functionBody
    A string containing the JavaScript statements comprising the function definition.

    Invoking the Function constructor as a function (without using the new operator) has the same effect as invoking it as a constructor.

    Function parameters

    Note: Default and rest parameters are experimental technology, part of the ECMAScript 6 proposal, and are not widely supported by browsers yet.

    Default parameters

    Default function parameters allow formal parameters to be initialized with default values if no value or undefined is passed. For more details, see default parameters.

    Rest parameters

    The rest parameter syntax allows to represent an indefinite number of arguments as an array. For more details, see rest parameters.

    The arguments object

    You can refer to a function's arguments within the function by using the arguments object. See arguments.

    • arguments: An array-like object containing the arguments passed to the currently executing function.
    • arguments.callee : The currently executing function.
    • arguments.caller : The function that invoked the currently executing function.
    • arguments.length: The number of arguments passed to the function.

    Defining method functions

    Getter and setter functions

    You can define getters (accessor methods) and setters (mutator methods) on any standard built-in object or user-defined object that supports the addition of new properties. The syntax for defining getters and setters uses the object literal syntax.

    get

    Binds an object property to a function that will be called when that property is looked up.

    set
    Binds an object property to a function to be called when there is an attempt to set that property.

    Method definition syntax

    Note: Method definitions are experimental technology, part of the ECMAScript 6 proposal, and are not widely supported by browsers yet.

    Starting with ECMAScript 6, you are able to define own methods in a shorter syntax, similar to the getters and setters. See method definitions for more information.

    var obj = {
      foo() {},
      bar() {}
    };

    Function constructor vs. function declaration vs. function expression

    Compare the following:

    A function defined with the Function constructor assigned to the variable multiply

    function multiply(x, y) {
       return x * y;
    }
    

    A function expression of an anonymous function assigned to the variable multiply

    var multiply = function(x, y) {
       return x * y;
    };
    

    A function expression of a function named func_name assigned to the variable multiply

    var multiply = function func_name(x, y) {
       return x * y;
    };
    

    Differences

    All do approximately the same thing, with a few subtle differences:

    There is a distinction between the function name and the variable the function is assigned to. The function name cannot be changed, while the variable the function is assigned to can be reassigned. The function name can be used only within the function's body. Attempting to use it outside the function's body results in an error (or undefined if the function name was previously declared via a var statement). For example:

    var y = function x() {};
    alert(x); // throws an error
    

    The function name also appears when the function is serialized via Function's toString method.

    On the other hand, the variable the function is assigned to is limited only by its scope, which is guaranteed to include the scope where the function is declared in.

    As the 4th example shows, the function name can be different from the variable the function is assigned to. They have no relation to each other.A function declaration also creates a variable with the same name as the function name. Thus, unlike those defined by function expressions, functions defined by function declarations can be accessed by their name in the scope they were defined in:

    A function defined by 'new Function' does not have a function name. However, in the SpiderMonkey JavaScript engine, the serialized form of the function shows as if it has the name "anonymous". For example, alert(new Function()) outputs:

    function anonymous() {
    }
    

    Since the function actually does not have a name, anonymous is not a variable that can be accessed within the function. For example, the following would result in an error:

    var foo = new Function("alert(anonymous);");
    foo();
    

    Unlike functions defined by function expressions or by the Function constructor, a function defined by a function declaration can be used before the function declaration itself. For example:

    foo(); // alerts FOO!
    function foo() {
       alert('FOO!');
    }
    

    A function defined by a function expression inherits the current scope. That is, the function forms a closure. On the other hand, a function defined by a Function constructor does not inherit any scope other than the global scope (which all functions inherit).

    Functions defined by function expressions and function declarations are parsed only once, while those defined by the Function constructor are not. That is, the function body string passed to the Function constructor must be parsed each and every time the constructor is called. Although a function expression creates a closure every time, the function body is not reparsed, so function expressions are still faster than "new Function(...)". Therefore the Function constructor should generally be avoided whenever possible.

    It should be noted, however, that function expressions and function declarations nested within the function generated by parsing a Function constructor 's string aren't parsed repeatedly. For example:

    var foo = (new Function("var bar = \'FOO!\';\nreturn(function() {\n\talert(bar);\n});"))();
    foo(); // The segment "function() {\n\talert(bar);\n}" of the function body string is not re-parsed.

    A function declaration is very easily (and often unintentionally) turned into a function expression. A function declaration ceases to be one when it either:

    • becomes part of an expression
    • is no longer a "source element" of a function or the script itself. A "source element" is a non-nested statement in the script or a function body:
    var x = 0;               // source element
    if (x == 0) {            // source element
       x = 10;               // not a source element
       function boo() {}     // not a source element
    }
    function foo() {         // source element
       var y = 20;           // source element
       function bar() {}     // source element
       while (y == 10) {     // source element
          function blah() {} // not a source element
          y++;               // not a source element
       }
    }
    

    Examples

    // function declaration
    function foo() {}
    
    // function expression
    (function bar() {})
    
    // function expression
    x = function hello() {}
    
    
    if (x) {
       // function expression
       function world() {}
    }
    
    
    // function declaration
    function a() {
       // function declaration
       function b() {}
       if (0) {
          // function expression
          function c() {}
       }
    }
    

    Conditionally defining a function

    Functions can be conditionally defined using either //function statements// (an allowed extension to the ECMA-262 Edition 3 standard) or the Function constructor. Please note that such function statements are no longer allowed in ES5 strict. Additionally, this feature does not work consistently cross-browser, so you should not rely on it.

    In the following script, the zero function is never defined and cannot be invoked, because 'if (0)' evaluates its condition to false:

    if (0) {
       function zero() {
          document.writeln("This is zero.");
       }
    }
    

    If the script is changed so that the condition becomes 'if (1)', function zero is defined.

    Note: Although this kind of function looks like a function declaration, it is actually an expression (or statement), since it is nested within another statement. See differences between function declarations and function expressions.

    Note: Some JavaScript engines, not including SpiderMonkey, incorrectly treat any function expression with a name as a function definition. This would lead to zero being defined, even with the always-false if condition. A safer way to define functions conditionally is to define the function anonymously and assign it to a variable:

    if (0) {
       var zero = function() {
          document.writeln("This is zero.");
       }
    }
    

    Examples

    Returning a formatted number

    The following function returns a string containing the formatted representation of a number padded with leading zeros.

    // This function returns a string padded with leading zeros
    function padZeros(num, totalLen) {
       var numStr = num.toString();             // Initialize return value as string
       var numZeros = totalLen - numStr.length; // Calculate no. of zeros
       for (var i = 1; i <= numZeros; i++) {
          numStr = "0" + numStr;
       }
       return numStr;
    }
    

    The following statements call the padZeros function.

    var result;
    result = padZeros(42,4); // returns "0042"
    result = padZeros(42,2); // returns "42"
    result = padZeros(5,4);  // returns "0005" 
    

    Determining whether a function exists

    You can determine whether a function exists by using the typeof operator. In the following example, a test is peformed to determine if the window object has a property called noFunc that is a function. If so, it is used; otherwise some other action is taken.

     if ('function' == typeof window.noFunc) {
       // use noFunc()
     } else {
       // do something else
     }
    

    Note that in the if test, a reference to noFunc is used—there are no brackets "()" after the function name so the actual function is not called.

    Specifications

    Specification Status Comment
    ECMAScript 1st Edition. Standard Initial definition. Implemented in JavaScript 1.0
    ECMAScript 5.1 (ECMA-262)
    The definition of 'Function Definition' in that specification.
    Standard  
    ECMAScript 6 (ECMA-262)
    The definition of 'Function definitions' in that specification.
    Release Candidate New: Arrow functions, Generator functions, default parameters, rest parameters
    ECMAScript 6 (ECMA-262)
    The definition of 'function*' in that specification.
    Release Candidate Initial definition.
    ECMAScript 6 (ECMA-262)
    The definition of 'Arrow Function Definitions' in that specification.
    Release Candidate Initial definition.

    Browser compatibility

    Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
    Basic support (Yes) (Yes) (Yes) (Yes) (Yes)
    Generator function 39 26.0 (26.0) ? 26 ?
    Arrow function Not supported 22.0 (22.0) Not supported Not supported Not supported
    Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
    Basic support (Yes) (Yes) (Yes) (Yes) (Yes) (Yes)
    Generator function ? 39 26.0 (26.0) ? 26 ?
    Arrow function Not supported Not supported 22.0 (22.0) Not supported Not supported Not supported

    See also

    Document Tags and Contributors

    Contributors to this page: fscholz, sharq
    Last updated by: fscholz,
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