Question

Are these two functions doing the same thing behind the scenes? (in single statement functions)

var evaluate = function(string) {
    return eval('(' + string + ')');
}

var func = function(string) {
    return (new Function( 'return (' + string + ')' )());
}

console.log(evaluate('2 + 1'));
console.log(func('2 + 1'));

Answer 1

No, they are not the same.

• eval() evaluates a string as a JavaScript expression within the current execution scope and can access local variables.
• new Function() parses the JavaScript code stored in a string into a function object, which can then be called. It cannot access local variables because the code runs in a separate scope.

Consider this code:

function test1() {
    var a = 11;
    eval('(a = 22)');
    alert(a);            // alerts 22
}

If new Function('return (a = 22);')() were used, the local variable a would retain its value. Nevertheless, some JavaScript programmers such as Douglas Crockford believe that neither should be used unless absolutely necessary, and evaling/using the Function constructor on untrusted data is insecure and unwise.