JS Function to filter array of objects

objArrUniqKeys.js

function objArrUniqKeys(arr, key) {
  if(arr instanceof Array) {
    var keys = [];
    return arr.filter(function(elem, idx, arr) {
      if(typeof elem == "object") {
        if(elem.hasOwnProperty(key) && keys.indexOf(elem[key]) == -1) {
          keys.push(elem[key]);
          return true;
        }
      }
      return false;
    });
  }
  return arr;
}

var a = [{ id: 1 }, { id: 2 }, { id: 3 }, { id: 1 }, { id: 3 }];
a = objArrUniqKeys(a, "id");