eval() vs. Function() in JavaScript

We will learn about JavaScript functions eval() and Function(). The eval() and Function() are used to evaluate any JavaScript expression passed to either of them as a string but the difference between them is how how they handle the expression.

eval() 

The eval() method in JavaScript evaluates or executes its argument. If the argument is an expression, it evaluates the expression. If it’s one or more JavaScript statements, eval() executes the statements.

Syntax:

eval(String)

Example: In this example, we use `eval()` to evaluate the multiplication expression of `a` and `b`, converting the result to a string.

JavaScript

// JavaScript to illustrate eval() function
function func() {
 
    // Original string
    let a = 4;
    let b = 4;
 
    // Finding the multiplication
    let value = eval(new String(a * b));
    console.log(value);
}
// Driver code
func();

Output

[String: '16']

Function()

The Function() constructor creates a new function object dynamically from a string representing JavaScript code. It allows us to define functions programmatically at runtime. The code inside the string is compiled when the function is created, not when it is called.

Syntax:

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

Example: In this example, we use the Function() constructor to create a new function that multiplies two numbers.

JavaScript

// JavaScript to illustrate Function() constructor

// Define a string containing JavaScript code
const code = 'return 4 * 4;';

// Create a new function using Function() constructor
const multiply = new Function(code);

// Execute the dynamically created function
const result = multiply();
console.log(result); // Output: 16

Output

16

Difference between eval() vs. Function() in JavaScript

Feature	eval()	Function()
Usage	Evaluates JavaScript code as a string	Creates a new function from a string of code
Scope	Runs code in the current lexical scope	Creates a new function with its own lexical scope
Performance	Generally slower due to parsing and execution	Can be faster since code is compiled upfront
Security	Considered risky due to potential security issues	Generally safer as it creates a new function object
Dynamic code	Suitable for executing dynamic code	Suitable for generating functions dynamically
Syntax Errors	May not throw syntax errors immediately	Throws syntax errors at creation time
Syntax	eval(String)	new Function([arg1], functionBody)