JavaScript: Function Constructor

By Xah Lee. Date: . Last updated: .

Function lets you define function from strings.

Function(param_name_1, param_name_2, param_name_3 etc, body)
Return a function with specified params and body. The arguments should all be strings.
new Function (
same as Function (

The advantage of using Function() to define a function is that it lets you create function at run-time. Though, this is rarely needed.


const f = Function("a", "b", "return a + b;");

// function with no parameter
const h1 = new Function ("return 3;");
console.log( h1() );              // 3
// function with 1 parameter
const h2 = new Function("a", "return a;");
console.log(h2(4));             // 4
// function with 2 parameters
const h3 = new Function("a", "b", "return a + b;");
console.log(h3(3,4));           // 7

Using new Function() is similar to the use of eval(
), and you should be careful to not eval user generated input. Otherwise, it's a security risk.

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