Restricting eval() to a narrow scope

I have a javascript file that reads another file which may contain javascript fragments that need to be eval()-ed. The script fragments are supposed to conform to a strict subset of javascript that limits what they can do and which variables they can change, but I want to know if there is some way to enforce this by preventing the eval from seeing variables in the global scope. Something like the following:

function safeEval( fragment )
{
    var localVariable = g_Variable;

    {
        // do magic scoping here so that the eval fragment can see localVariable
        // but not g_Variable or anything else outside function scope

        eval( fragment );
    }
}

The actual code doesn't need to look like this--I'm open to any and all weird tricks with closures, etc. But I do want to know if this is even possible.

Short answer: No. If it's in the global scope, it's available to anything.

Long answer: if you're eval()ing untrusted code that really wants to read or mess with your execution environment, you're screwed. But if you own and trust all code being executed, including that being eval()ed, you can fake it by overriding the execution context:

function maskedEval(scr)
{
    // set up an object to serve as the context for the code
    // being evaluated. 
    var mask = {};
    // mask global properties 
    for (p in this)
        mask[p] = undefined;

    // execute script in private context
    (new Function( "with(this) { " + scr + "}")).call(mask);
}

Again, I must stress:

This will only serve to shield trusted code from the context in which it is executed. If you don't trust the code, DO NOT eval() it (or pass it to new Function(), or use it in any other way that behaves like eval()).

Shog9♦'s Answer is great. But if your code is just an expression, the code will be executed and nothing will be returned. For expressions, use

function evalInContext(context, js) {

  return eval('with(context) { ' + js + ' }');

}

Here is how to use it:

var obj = {key: true};

evalInContext(obj, 'key ? "YES" : "NO"');

It will return "YES".

If you are not sure if the code to be executed is expressions or statements, you can combine them:

function evalInContext(context, js) {

  var value;

  try {
    // for expressions
    value = eval('with(context) { ' + js + ' }');
  } catch (e) {
    if (e instanceof SyntaxError) {
      try {
        // for statements
        value = (new Function('with(this) { ' + js + ' }')).call(context);
      } catch (e) {}
    }
  }

  return value;
}

Similar to the dynamic function wrapping script in a with block approach above, this allows you to add pseudo-globals to the code you want to execute. You can "hide" specific things by adding them to the context.

function evalInContext(source, context) {
    source = '(function(' + Object.keys(context).join(', ') + ') {' + source + '})';

    var compiled = eval(source);

    return compiled.apply(context, values());

    // you likely don't need this - use underscore, jQuery, etc
    function values() {
        var result = [];
        for (var property in context)
            if (context.hasOwnProperty(property))
                result.push(context[property]);
        return result;
    }
}

See http://jsfiddle.net/PRh8t/ for an example. Note that Object.keys is not supported in all browsers.

You cant limit the scope of eval

btw see this post

There may be some other way to accomplish what it is you want accomplish in the grand scheme of things but you cannot limit the scope of eval in any way. You may be able to hide certain variables as pseudo private variables in javascript, but I dont think this is what you're going for.

There is a project called Google Caja. You can "sandbox" third party javascript using Caja. https://developers.google.com/caja/

Restricting eval() to a narrow scope

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