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The Function constructor creates a new Function object. Calling the constructor directly can create functions dynamically, but suffers from security and similar (but far less significant) performance issues similar to eval. However, unlike eval, the Function constructor allows executing code in the global scope, prompting better programming habits and allower for more efficient code minification.

Every JavaScript function is actually a Function object. This can be seen with the code (function(){}).constructor === Function which returns true.

Syntax

new Function ([arg1[, arg2[, ...argN]],] functionBody)

Parameters

arg1, arg2, ... argN
Names to be used by the function as formal argument names. Each must be a string that corresponds to 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.

Description

Function objects created with the Function constructor are parsed when the function is created. This is less efficient than declaring a function with a function expression or function statement and calling it within your code because such functions are parsed with the rest of the code.

All arguments passed to the function are treated as the names of the identifiers of the parameters in the function to be created, in the order in which they are passed.

Invoking the Function constructor as a function (without using the new operator) has the same effect as invoking it as a constructor. However, getting rid of the new operator allows for a smaller minified code size (4 bytes smaller), so it is best to not use new with Function.

Properties and Methods of Function

The global Function object has no methods or properties of its own. However, since it is a function itself, it does inherit some methods and properties through the prototype chain from Function.prototype.

Function prototype object

Properties

Function.prototype.arguments
An array corresponding to the arguments passed to a function. This is deprecated as property of Function. Use the arguments object available within the function instead.
Function.arity
Used to specifiy the number of arguments expected by the function, but has been removed. Use the length property instead.
Function.prototype.caller
Specifies the function that invoked the currently executing function.
Function.prototype.length
Specifies the number of arguments expected by the function.
Function.prototype.name
The name of the function.
Function.displayName
The display name of the function.
Function.prototype.constructor
Specifies the function that creates an object's prototype. See Object.prototype.constructor for more details.

Methods

Function.prototype.apply()
Calls a function and sets its this to the provided value, arguments can be passed as an Array object.
Function.prototype.bind()
Creates a new function which, when called, has its this set to the provided value, with a given sequence of arguments preceding any provided when the new function was called.
Function.prototype.call()
Calls (executes) a function and sets its this to the provided value, arguments can be passed as they are.
Function.prototype.isGenerator()
Returns true if the function is a generator; otherwise returns false.
Function.prototype.toSource()
Returns a string representing the source code of the function. Overrides the Object.prototype.toSource method.
Function.prototype.toString()
Returns a string representing the source code of the function. Overrides the Object.prototype.toString method.

Function instances

Function instances inherit methods and properties from Function.prototype. As with all constructors, you can change the constructor's prototype object to make changes to all Function instances.

Examples

Specifying arguments with the Function constructor

The following code creates a Function object that takes two arguments.

// Example can be run directly in your JavaScript console

// Create a function that takes two arguments and returns the sum of those arguments
var adder = new Function('a', 'b', 'return a + b');

// Call the function
adder(2, 6);
// > 8

The arguments "a" and "b" are formal argument names that are used in the function body, "return a + b".

Difference between Function constructor and function declaration

Functions created with the Function constructor do not create closures to their creation contexts; they always are created in the global scope. When running them, they will only be able to access their own local variables and global ones, not the ones from the scope in which the Function constructor was called. This is different from using eval with code for a function expression.

var x = 10;

function createFunction1() {
    var x = 20;
    return new Function('return x;'); // this |x| refers global |x|
}

function createFunction2() {
    var x = 20;
    function f() {
        return x; // this |x| refers local |x| above
    }
    return f;
}

var f1 = createFunction1();
console.log(f1());          // 10
var f2 = createFunction2();
console.log(f2());          // 20

The "proper" way to execute external code with Function (for maximum minifyability).

function makeFunction(code){
    return Function('"use strict";return ' + code)();
}
var add = makeFunction(
  "" + function(a, b, c){ return a + b + c } // move this to a separate file in the production release
);
console.log( add(1, 2, 3) ); // will log six

Please note that the above code by itself is completely impractical. You should never abuse Function like that. Instead, the above code is meant only to be a simplified example of something like a module loader where there is a base script, then there are hundreads of big optionally loaded modules. Then, instead of the user waiting while they all download, the clients computer only downloads modules as needed so the page loads super fast. Also, it is reccomended that when evaluating many functions, the functions are evaluated together in bulk instead of separatly.

function bulkMakeFunctions(){
    var str = "", i = 1, Len = arguments.length;
    if (Len) {
        str = arguments[0];
        while (i !== Len) str += "," + arguments[i], ++i;
    }
    return Function('"use strict";return[' + str + ']')();
}
const [
    add,                        sub,                        mul,                        div
] = bulkMakeFunctions(
    "function(a,b){return a+b}","function(a,b){return a-b}","function(a,b){return a*b}","function(a,b){return a/b}"
);
console.log(sub(add(mul(4,3), div(225,5)), 7))

Specifications

Specification Status Comment
ECMAScript 1st Edition (ECMA-262) Standard Initial definition. Implemented in JavaScript 1.0.
ECMAScript 5.1 (ECMA-262)
The definition of 'Function' in that specification.
Standard  
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'Function' in that specification.
Standard  
ECMAScript Latest Draft (ECMA-262)
The definition of 'Function' in that specification.
Draft  

Browser compatibility

Update compatibility data on GitHub
DesktopMobileServer
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidEdge MobileFirefox for AndroidOpera for AndroidiOS SafariSamsung InternetNode.js
Basic supportChrome Full support YesEdge Full support YesFirefox Full support 1IE Full support YesOpera Full support YesSafari Full support YesWebView Android Full support YesChrome Android Full support YesEdge Mobile Full support YesFirefox Android Full support 4Opera Android Full support YesSafari iOS Full support YesSamsung Internet Android Full support Yesnodejs Full support Yes
arguments
Deprecated
Chrome Full support YesEdge Full support YesFirefox Full support 1IE Full support YesOpera Full support YesSafari Full support YesWebView Android Full support YesChrome Android Full support YesEdge Mobile Full support YesFirefox Android Full support 4Opera Android Full support YesSafari iOS Full support YesSamsung Internet Android Full support Yesnodejs Full support Yes
arity
DeprecatedNon-standard
Chrome No support NoEdge No support NoFirefox No support NoIE No support NoOpera No support NoSafari No support NoWebView Android No support NoChrome Android No support NoEdge Mobile No support NoFirefox Android No support NoOpera Android No support NoSafari iOS No support NoSamsung Internet Android No support Nonodejs No support No
caller
Non-standard
Chrome Full support YesEdge Full support YesFirefox Full support 1IE Full support 8Opera Full support YesSafari Full support YesWebView Android Full support YesChrome Android Full support YesEdge Mobile Full support YesFirefox Android Full support 4Opera Android Full support YesSafari iOS Full support YesSamsung Internet Android Full support Yesnodejs Full support Yes
displayName
Non-standard
Chrome ? Edge ? Firefox Full support 13IE ? Opera ? Safari ? WebView Android ? Chrome Android ? Edge Mobile ? Firefox Android Full support 14Opera Android ? Safari iOS ? Samsung Internet Android ? nodejs ?
lengthChrome Full support YesEdge Full support YesFirefox Full support 1IE Full support YesOpera Full support YesSafari Full support YesWebView Android Full support YesChrome Android Full support YesEdge Mobile Full support YesFirefox Android Full support 4Opera Android Full support YesSafari iOS Full support YesSamsung Internet Android Full support Yesnodejs Full support Yes
nameChrome Full support 15Edge Full support 14Firefox Full support 1IE No support NoOpera Full support YesSafari Full support YesWebView Android Full support YesChrome Android Full support YesEdge Mobile Full support YesFirefox Android Full support 4Opera Android Full support YesSafari iOS Full support YesSamsung Internet Android Full support Yesnodejs Full support Yes
prototypeChrome Full support YesEdge Full support YesFirefox Full support 1IE Full support YesOpera Full support YesSafari Full support YesWebView Android Full support YesChrome Android Full support YesEdge Mobile Full support YesFirefox Android Full support 4Opera Android Full support YesSafari iOS Full support YesSamsung Internet Android Full support Yesnodejs Full support Yes
applyChrome Full support YesEdge Full support YesFirefox Full support 1IE Full support YesOpera Full support YesSafari Full support YesWebView Android Full support YesChrome Android Full support YesEdge Mobile Full support YesFirefox Android Full support 4Opera Android Full support YesSafari iOS Full support YesSamsung Internet Android Full support Yesnodejs Full support Yes
bindChrome Full support 7Edge Full support YesFirefox Full support 4IE Full support 9Opera Full support 11.6Safari Full support 5.1WebView Android Full support 4Chrome Android Full support 18Edge Mobile Full support YesFirefox Android Full support 4Opera Android Full support 11.5Safari iOS Full support 6Samsung Internet Android Full support Yesnodejs Full support Yes
callChrome Full support YesEdge Full support YesFirefox Full support 1IE Full support YesOpera Full support YesSafari Full support YesWebView Android Full support YesChrome Android Full support YesEdge Mobile Full support YesFirefox Android Full support 4Opera Android Full support YesSafari iOS Full support YesSamsung Internet Android Full support Yesnodejs Full support Yes
isGenerator
DeprecatedNon-standard
Chrome No support NoEdge No support NoFirefox No support 5 — 58IE No support NoOpera No support NoSafari No support NoWebView Android No support NoChrome Android No support NoEdge Mobile No support NoFirefox Android No support 5 — 58Opera Android No support NoSafari iOS No support NoSamsung Internet Android No support Nonodejs No support No
toSource
Non-standard
Chrome No support NoEdge No support NoFirefox Full support 1IE No support NoOpera No support NoSafari No support NoWebView Android No support NoChrome Android No support NoEdge Mobile No support NoFirefox Android Full support 4Opera Android No support NoSafari iOS No support NoSamsung Internet Android No support Nonodejs No support No
toStringChrome Full support YesEdge Full support YesFirefox Full support 1IE Full support 5Opera Full support YesSafari Full support YesWebView Android Full support YesChrome Android Full support YesEdge Mobile Full support YesFirefox Android Full support 4Opera Android Full support YesSafari iOS Full support YesSamsung Internet Android Full support Yesnodejs Full support Yes

Legend

Full support  
Full support
No support  
No support
Compatibility unknown  
Compatibility unknown
Non-standard. Expect poor cross-browser support.
Non-standard. Expect poor cross-browser support.
Deprecated. Not for use in new websites.
Deprecated. Not for use in new websites.

See also

Document Tags and Contributors

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