Easy way to evaluate path-like expressions in Javascript?

Question

If I have a JavaScript object such as:

var x = {foo: 42, bar: {fubar: true}}

then I can get the value true with var flag = x.bar.fubar. I'd like to be able to separate out and store the path "bar.fubar", then evaluate it dynamically. Something like:

var paths = ["bar.fubar", ...];
...
var flag = evalPath( x, paths[0] );

Obviously I could write a simple parser and evaluator for a basic path expression grammar. But under DRY principles I wonder if there's already an existing way to do something like evalPath built-in to JavaScript, or a small library that would do the job? I also anticipate needing array indexes in the path expression in future.

Update Just to be clear, I'm not asking for code samples - my question is whether there's existing code (built-in or library) I can re-use. Thanks to the contributors below for suggestions of code samples anyway! Note that none of them handle the array index requirement.

Answer 1

JavaScript provides eval, but I don't recommend it.

Answer 2

You could try something like this. I can't really think of a situation where it would be appropriate to store paths this way though.

function evalPath(obj, path) {
  var pathLevels = path.split('.');
  var result = obj;
  for (var i = 0; i < pathLevels.length; i++) {
    try {
      result = result[pathLevels[i]];
    }
    catch (e) {
      alert('Property not found:' + pathLevels[i]);
    }
  }
  return result;
}

The alert is really only there for debugging purposes. You may want to return null or something.

Answer 3

Why not try something like

function evalPath(obj, path)
{
    var rtnValue = obj;

    // Split our path into an array we can iterate over
    var path = path.split(".");
    for (var i = 0, max=path.length; i < max; i++) 
    {
        // If setting current path to return value fails, set to null and break
        if (typeof (rtnValue = rtnValue[path[i]]) == "undefined")
        {
            rtnValue = null;
            break;
        }
    }

    // Return the final path value, or null if it failed
    return rtnValue;
}

Not tested, but it should work fairly well. Like XPath, it will return null if it can't find what it's looking for.

Answer 4

like

function locate(obj, path) {
    var p = path.split("."), a = p.shift();
    if(a in obj)
       return p.length ? locate(obj[a], p.join(".")) : obj[a];
     return undefined;
}

locate(x, "bar.fubar")

this works on the right only, of course

Answer 5

Doing a quick search, I came across JSONPath. Haven't used it at all, but it looks like it might do what you want it to.

Example usage:

var x = {foo: 42, bar: {fubar: true}}
var res1 = jsonPath(x, "$.bar.fubar"); // Array containing fubar's value