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);

Output as tested in node.js

[ [ 'a', 'b', 'c' ],
  [ 'a', 'b', 'c' ],
  [ 'a', 'b', 'c' ],
  [ 'a', 'b', 'c' ],
  [ 'a', 'b', 'c' ] ]

I prefer the closure style, but knowing what I know about how V8 optimized functions, the one with explicit state passing might be faster. Benchmark both to find out.

Turns out the closure version is only slightly slower. I'd prefer it since it's much cleaner code to me. http://jsperf.com/get-keys-comprehensive