101/assign.js

common javascript utils that can be required selectively that assume es5+

/**
 * @module 101/assign
 */

/**
 * Copies enumerable and own properties from a source object(s) to a target object, aka extend.
 * https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/assign
 * I added functionality to support assign as a partial function
 * @function module:101/assign
 * @param {object} [target] - object which source objects are extending (being assigned to)
 * @param {object} sources... - objects whose properties are being assigned to the source object
 * @return {object} source with extended properties
 */
module.exports = assign;

function assign (target, firstSource) {
  if (arguments.length === 1) {
    firstSource = arguments[0];
    return function (target) {
      return assign(target, firstSource);
    };
  }
  if (target === undefined || target === null)
    throw new TypeError('Cannot convert first argument to object');
  var to = Object(target);
  for (var i = 1; i < arguments.length; i++) {
    var nextSource = arguments[i];
    if (nextSource === undefined || nextSource === null) continue;
    var keysArray = Object.keys(Object(nextSource));
    for (var nextIndex = 0, len = keysArray.length; nextIndex < len; nextIndex++) {
      var nextKey = keysArray[nextIndex];
      Object.getOwnPropertyDescriptor(nextSource, nextKey);
      // I changed the following line to get 100% test coverage.
      // if (desc !== undefined && desc.enumerable) to[nextKey] = nextSource[nextKey];
      // I was unable to find a scenario where desc was undefined or that desc.enumerable was false:
      //   1) Object.defineProperty does not accept undefined as a desc
      //   2) Object.keys does not return non-enumerable keys.
      // Let me know if this is a cross browser thing.
      to[nextKey] = nextSource[nextKey];
    }
  }
  return to;
}