const shallowequal = require("shallowequal");

const object = { user: "fred" };
const other = { user: "fred" };

object == other;
// → false

shallowequal(object, other);
// → true

module.exports = function shallowEqual(objA, objB, compare, compareContext) {

  // 如果用户自定义了比较函数 compare，直接使用并返回
  var ret = compare ? compare.call(compareContext, objA, objB) : void 0;

  if (ret !== void 0) {
    return !!ret;
  }

  // https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Object/is]
  // 比较两个值是否相等，简单类型相等，或者复杂类型(对象)内存相等
  if (Object.is(objA, objB)) {
    return true;
  }

  // 如果不是对象，不等于
  if (typeof objA !== "object" || !objA || typeof objB !== "object" || !objB) {
    return false;
  }

  // 获取对象的键数组
  var keysA = Object.keys(objA);
  var keysB = Object.keys(objB);

  // 先比较键的长度
  if (keysA.length !== keysB.length) {
    return false;
  }

  var bHasOwnProperty = Object.prototype.hasOwnProperty.bind(objB);

  // Test for A's keys different from B.
  // 判断两个对象的每一个键值对是否相等
  for (var idx = 0; idx < keysA.length; idx++) {
    var key = keysA[idx];

    if (!bHasOwnProperty(key)) {
      return false;
    }

    var valueA = objA[key];
    var valueB = objB[key];

    ret = compare ? compare.call(compareContext, valueA, valueB, key) : void 0;

    if (ret === false || (ret === void 0 && !Object.is(valueA, valueB))) {
      return false;
    }
  }

  return true;
};