Evaluating JavaScript

Introduction#

In JavaScript, the eval function evaluates a string as if it were JavaScript code. The return value is the result of the evaluated string, e.g. eval('2 + 2') returns 4.

eval is available in the global scope. The lexical scope of the evaluation is the local scope unless invoked indirectly (e.g. var geval = eval; geval(s);).

The use of eval is strongly discouraged. See the Remarks section for details.

Syntax#

eval(string);

Parameters#

Parameter	Details
string	The JavaScript to be evaluated.

Remarks#

The use of eval is strongly discouraged; in many scenarios it presents a security vulnerability.

eval() is a dangerous function, which executes the code it’s passed with the privileges of the caller. If you run eval() with a string that could be affected by a malicious party, you may end up running malicious code on the user’s machine with the permissions of your webpage / extension. More importantly, third party code can see the scope in which eval() was invoked, which can lead to possible attacks in ways to which the similar Function is not susceptible.

MDN JavaScript Reference

Additionally:

Introduction

You can always run JavaScript from inside itself, although this is strongly discouraged due to the security vulnerabilities it presents (see Remarks for details).

To run JavaScript from inside JavaScript, simply use the below function:

eval("var a = 'Hello, World!'");

Evaluation and Math

You can set a variable to something with the eval() function by using something similar to the below code:

var x = 10;
var y = 20;
var a = eval("x * y") + "<br>";
var b = eval("2 + 2") + "<br>";
var c = eval("x + 17") + "<br>";

var res = a + b + c;

The result, stored in the variable res, will be:

200
4
27

The use of eval is strongly discouraged. See the Remarks section for details.

Evaluate a string of JavaScript statements

var x = 5;
var str = "if (x == 5) {console.log('z is 42'); z = 42;} else z = 0; ";

console.log("z is ", eval(str));

The use of eval is strongly discouraged. See the Remarks section for details.