Key extractor that supports nested objects, nested arrays, duplicate keys, and cycles, returning an array of the unique keys.

extract-closure.js

// All of these should return the keys "a", "b", "c"

var inputs = [
  {a:1,b:2,c:3}, // Simple object
  {a:{b:2,c:3}}, // Simple object with nesting
  {a:{a:{b:2,c:3}}}, // Repeated key hiding nesting
  {a:[{b:2,c:3}]}, // keys behind array
];
inputs.push(inputs); // reference cycle and array at top

function getKeys(obj) {
  var all = {};
  var seen = [];
  checkValue(obj);
  return Object.keys(all);
  function checkValue(value) {
    if (Array.isArray(value)) return checkArray(value);
    if (value instanceof Object) return checkObject(value);
  }
  function checkArray(array) {
    if (seen.indexOf(array) >= 0) return;
    seen.push(array);
    for (var i = 0, l = array.length; i < l; i++) {
      checkValue(array[i]);
    }
  }
  function checkObject(obj) {
    if (seen.indexOf(obj) >= 0) return;
    seen.push(obj);
    var keys = Object.keys(obj);
    for (var i = 0, l = keys.length; i < l; i++) {
      var key = keys[i];
      all[key] = true;
      checkValue(obj[key]);
    }
  }
}


var result = inputs.map(getKeys);
console.log(result);

extract.js

// All of these should return the keys "a", "b", "c"

var inputs = [
  {a:1,b:2,c:3}, // Simple object
  {a:{b:2,c:3}}, // Simple object with nesting
  {a:{a:{b:2,c:3}}}, // Repeated key hiding nesting
  {a:[{b:2,c:3}]}, // keys behind array
];
inputs.push(inputs); // reference cycle and array at top

function getKeys(obj) {
  var all = {};
  var seen = [];
  checkValue(obj, all, seen);
  return Object.keys(all);
}

function checkValue(value, all, seen) {
  if (Array.isArray(value)) return checkArray(value, all, seen);
  if (value instanceof Object) return checkObject(value, all, seen);
}
function checkArray(array, all, seen) {
  if (seen.indexOf(array) >= 0) return;
  seen.push(array);
  for (var i = 0, l = array.length; i < l; i++) {
    checkValue(array[i], all, seen);
  }
}
function checkObject(obj, all, seen) {
  if (seen.indexOf(obj) >= 0) return;
  seen.push(obj);
  var keys = Object.keys(obj);
  for (var i = 0, l = keys.length; i < l; i++) {
    var key = keys[i];
    all[key] = true;
    checkValue(obj[key], all, seen);
  }
}

var result = inputs.map(getKeys);
console.log(result);