函數(shù)isEqual接收兩個(gè)對(duì)象, 判斷它們的相等性, 返回boolean

根據(jù)相等判斷的實(shí)用性, 做出如下的相等性說(shuō)明:

• 原始類型Number, String, Boolean, Undefined, Null: 直接通過(guò)===判斷, 經(jīng)過(guò)對(duì)象包裝(eg: new Number(1))的這類對(duì)象轉(zhuǎn)換為原始類型再判斷(即_.isEqual(new Number(1), 1)===true)

• 引用類型Object: 引用相同, 必然相等; 引用不同, 進(jìn)行深入比較(其中type為function的類型, 先判斷構(gòu)造函數(shù)是是否相等, 再判斷其自有的屬性和方法是否也一致)

原生js存在的不完善地方,這里列舉一些常用到的特例:

• 原生會(huì)返回true的情況:

• null == undefined(通過(guò)null === undefined返回false修正)

• -0 === +0(通過(guò)-Infinity === Infinity返回false修正)

• NaN !== NaN(通過(guò)if(a!==a) return (b!==b)修正)

• 原生會(huì)返回false

• 經(jīng)過(guò)對(duì)象包裝的原始類型的比較: new Number(1) === 1 (原生的這樣判斷其實(shí)是無(wú)可厚非的, 實(shí)際運(yùn)用中我們統(tǒng)一都?xì)w為相等)

用到的工具:

• typeof
• Object.prototype.toString
• 運(yùn)算符+可以把字符串轉(zhuǎn)為數(shù)字
• lodash的keys,has方法

使用例子:

isEqual({name: 'rose'}, {name: 'rose'}) // 返回true
isEqual([{name: 'rose'}, 2], [{name: 'rose'}, 2]) // 返回true
var A = function(){}
var B = new A
var C = new A
isEqual(B, C) //返回true

代碼：

import _ from 'lodash'
var toString = Object.prototype.toString

var isEqual = function (a, b, aStack, bStack) {
  //原始類型Number, String, Boolean

  // 0 === -0 使用 1 / 0 !== 1/ -0 排除

  if (a === b) return a !== 0 || 1 / a === 1 / b

  // null == undefined  返回true
  // null === undefined 返回false
  if (a == null || b == null) return a === b


  // isNaN !== isNaN 返回true
  if (a !== a) return b !== b

  var className = toString.call(a)
  if (className !== toString.call(b)) return false
  switch (className) {

    case '[object RegExp]':
    case '[object String]':
      return '' + a === '' + b

    case '[object Number]':
      if (+a !== +a) return +b !== +b
      return +a === 0 ? 1 / +a === 1 / b : +a === +b
    case '[object Date]':
    case '[object Boolean]':
      return +a === +b
    // todo case '[object Symbol]'
  }

  // 判斷a 是否為數(shù)組
  var areArrays = className === '[object Array]'
  if (!areArrays) {
    if (typeof a !== 'object' || typeof b !== 'object') return false

    // Objects with different constructors are not equivalent, but `Object`s or `Array`s
    // from different frames are.
    // 先判斷構(gòu)造函數(shù)是否相同
    var aCtor = a.constructor, bCtor = b.constructor
    if (aCtor !== bCtor) return false
  }


  aStack = aStack || []
  bStack = bStack || []


  var length = aStack.length
  while (length--) {
    // 避免循環(huán)引用導(dǎo)致的死循環(huán)
    if (aStack[length] === a) return bStack[length] === b
  }

  aStack.push(a)
  bStack.push(b)



  if (areArrays) {
    length = a.length
    if (length !== b.length) return false


    while (length--) {
      if (!isEqual(a[length], b[length], aStack, bStack)) return false
    }


  } else {
    var keys = _.keys(a), key
    length = keys.length
    if (_.keys(b).length !== length) return false;

    while (length--) {
      // Deep compare each member
      // 遞歸比較
      key = keys[length];
      if (!(_.has(b, key) && isEqual(a[key], b[key], aStack, bStack))) return false;
    }

  }

  aStack.pop()
  bStack.pop()
  return true
}
export {
  isEqual
}

todo:

• 去掉lodash的依賴， 寫一個(gè)從零開始的對(duì)象判斷
• 加入es6新類型數(shù)據(jù)結(jié)構(gòu)的判斷

參考鏈接:

• lodash
• underscore源碼分析