joint.all.js
function transform(object, callback, accumulator, thisArg) {
      var isArr = isArray(object);
      callback = baseCreateCallback(callback, thisArg, 4);

      if (accumulator == null) {
        if (isArr) {
          accumulator = [];
        } else {
          var ctor = object && object.constructor,
              proto = ctor && ctor.prototype;

          accumulator = createObject(proto);
        }
      }
      (isArr ? forEach : forOwn)(object, function(value, index, object) {
        return callback(accumulator, value, index, object);
      });
      return accumulator;
    }
function transform(object, callback, accumulator, thisArg) {
      var isArr = isArray(object);
      callback = baseCreateCallback(callback, thisArg, 4);

      if (accumulator == null) {
        if (isArr) {
          accumulator = [];
        } else {
          var ctor = object && object.constructor,
              proto = ctor && ctor.prototype;

          accumulator = createObject(proto);
        }
      }
      (isArr ? forEach : forOwn)(object, function(value, index, object) {
        return callback(accumulator, value, index, object);
      });
      return accumulator;
    }
An alternative to `_.reduce` this method transforms `object` to a new
`accumulator` object which is the result of running each of its elements
through a callback, with each callback execution potentially mutating
the `accumulator` object. The callback is bound to `thisArg` and invoked
with four arguments; (accumulator, value, key, object). Callbacks may exit
iteration early by explicitly returning `false`.

@static
@memberOf _
@category Objects
@param {Array|Object} collection The collection to iterate over.
@param {Function} [callback=identity] The function called per iteration.
@param {*} [accumulator] The custom accumulator value.
@param {*} [thisArg] The `this` binding of `callback`.
@returns {*} Returns the accumulated value.
@example

var squares = _.transform([1, 2, 3, 4, 5, 6, 7, 8, 9, 10], function(result, num) {
num *= num;
if (num % 2) {
return result.push(num) < 3;
}
});
// => [1, 9, 25]

var mapped = _.transform({ 'a': 1, 'b': 2, 'c': 3 }, function(result, num, key) {
result[key] = num * 3;
});
// => { 'a': 3, 'b': 6, 'c': 9 }