Function

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

Methods

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

Browser compatibility

BCD tables only load in the browser

See also