eval()

The much-maligned eval() function receives an upgrade in strict mode. The biggest change to eval() is that it will no longer create variables or functions in the containing context. For example:

// eval() used to create a variable
// Non-strict mode: Alert displays 10
// Strict mode: Throws an ReferenceError when alert(x) is called
function doSomething(){
 eval("let x = 10");
 alert(x);
}

When run in nonstrict mode, this code creates a local variable x in the function doSomething() and that value is then displayed using alert(). In strict mode, the call to eval() does not create the variable x inside of doSomething() and so the call to alert() throws a ReferenceError because x is undeclared.

Variables and functions can be declared inside of eval(), but they remain inside a special scope that is used while code is being evaluated and then destroyed once completed. So the following code works without any errors:

"use strict";
let result = eval("let x ...