How to create a function from a string in JavaScript ?

The task is to create a function from the string given in the format of the function. Here are a few approaches that are listed below:

• Using Function() Constructor
• Using eval() Method

Approach 1: Using Function() Constructor

• Use the Function() Constructor to create a function from the string.
• It accepts any number of arguments (in the form of string). The last one should be the body of the function.
• In this example, Only the body of the function is passed, returning a value.

Example 1: This example implements the above approach.

Javascript

let func = 'return "This is return value";';

// We can use 'func' as function
function funFromString() {
    let func2 = Function(func);

    // Now 'func' can be used as function
    console.log(func2());
}

funFromString();

Output

This is return value

Approach 2: Using eval() Method

• Use the eval() method to create a function from the string.
• It accepts the function in the form of a string and converts it to a JavaScript function.
• In this example, It takes 2 arguments and returns the sum of both numbers.

Example 2: This example uses the approach discussed above. 

Javascript

const str = "var func = function (a, b) { return a + b; };";

// Till this point we can use 'func' as function
function funFromString() {

    // Converting the string to function
    eval(str);

    console.log(func(2, 5));
}

funFromString();

Output

7