var someObject = {
'part1' : {
    'name': 'Part 1',
    'txt': 'example',
},
'part2' : {
    'name': 'Part 2',
    'size': '15',
    'qty' : '60'
},
'part3' : [
    {
        'name': 'Part 3A',
        'size': '10',
        'qty' : '20'
    }, {
        'name': '[value]',
        'size': '5',
        'qty' : '20'
    }, {
        'name': 'Part 3C',
        'size': '7.5',
        'qty' : '20'
    }
]};

getPath(obj, '[value]');

function getPath(obj, val, path) {
   path = path || "";
   var fullpath = "";
   for (var b in obj) {
      if (obj[b] === val) {
         return (path + "/" + b);
      }
      else if (typeof obj[b] === "object") {
         fullpath = getPath(obj[b], val, path + "/" + b) || fullpath;
      }
   }
   return fullpath;
}

console.log(getPath(testObj, '20'));      // Prints: /part3/2/qty
console.log(getPath(testObj, "[value]")); // Prints: /part3/1/name

function getPath(obj, value, path){
    path = path || [];
    if(obj instanceof Array || obj instanceof Object){
        //Search within children
        for(var i in obj){
           path.push(i); //push path, will be pop() if no found
            var subPath = getPath(obj[i], value, path);
            //We found nothing in children
            if(subPath instanceof Array){
                if(subPath.length == 1 && subPath[0]==i){
                  path.pop();
                }
            //We found something in children
            }else if(subPath instanceof Object){
               path = subPath;
               break;
            }
        }
    }else{
        //Match ?
        if(obj == value){
            return {finalPath:path};
        }else{
            //not ->backtrack
            path.pop();
            return false;
        }
    }
    return path;
}