Restricting eval() to a narrow scope

Question

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 s

Answer 1

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;
}