index.js

module.exports = extend;

/*
  var obj = {a: 3, b: 5};
  extend(obj, {a: 4, c: 8}); // {a: 4, b: 5, c: 8}
  obj; // {a: 4, b: 5, c: 8}

  var obj = {a: 3, b: 5};
  extend({}, obj, {a: 4, c: 8}); // {a: 4, b: 5, c: 8}
  obj; // {a: 3, b: 5}

  var arr = [1, 2, 3];
  var obj = {a: 3, b: 5};
  extend(obj, {c: arr}); // {a: 3, b: 5, c: [1, 2, 3]}
  arr.push(4);
  obj; // {a: 3, b: 5, c: [1, 2, 3, 4]}

  var arr = [1, 2, 3];
  var obj = {a: 3, b: 5};
  extend(true, obj, {c: arr}); // {a: 3, b: 5, c: [1, 2, 3]}
  arr.push(4);
  obj; // {a: 3, b: 5, c: [1, 2, 3]}

  extend({a: 4, b: 5}); // {a: 4, b: 5}
  extend({a: 4, b: 5}, 3); {a: 4, b: 5}
  extend({a: 4, b: 5}, true); {a: 4, b: 5}
  extend('hello', {a: 4, b: 5}); // throws
  extend(3, {a: 4, b: 5}); // throws
*/

function extend(/* [deep], obj1, obj2, [objn] */) {
  var args = [].slice.call(arguments);
  var deep = false;
  if (typeof args[0] == 'boolean') {
    deep = args.shift();
  }
  var result = args[0];
  if (isUnextendable(result)) {
    throw new Error('extendee must be an object');
  }
  var extenders = args.slice(1);
  var len = extenders.length;
  for (var i = 0; i < len; i++) {
    var extender = extenders[i];
    for (var key in extender) {
      if (Object.prototype.hasOwnProperty.call(extender, key)) {
        var value = extender[key];
        if (deep && isCloneable(value)) {
          var base = Array.isArray(value) ? [] : {};
          result[key] = extend(
            true,
            Object.prototype.hasOwnProperty.call(result, key) && !isUnextendable(result[key])
              ? result[key]
              : base,
            value
          );
        } else {
          result[key] = value;
        }
      }
    }
  }
  return result;
}

function isCloneable(obj) {
  return Array.isArray(obj) || {}.toString.call(obj) == '[object Object]';
}

function isUnextendable(val) {
  return !val || (typeof val != 'object' && typeof val != 'function');
}