eMFtQj hIPNtw jQMBWr 所以在JS的世界里, 很需要一种侵入性不那么强而且易读的异步回调解决方案, ES7的规范里, ECMAScript又提出了async/await, 使用这个方案仍然需要使用Promise来改写异步过程, 但是需要用then来改写执行过程, 而是可以像同步代码一样将结果直接作为执行结果返回, 可阅读性也非常好. 这个方案的 hYcTaL hctuh jMfvbc gFwJbt [object Object],[object Object],[object Object],[object Object] ikkyVr bPjPUf cofShm lmYuZf euExUQ hDyWCg hUVKYO [object Object] uQPrX ekvxDO bpqJVJ iGaadT iGXldS hOxXax emJCYl ddGdPh iBSbYp dwejLW esHzLO"> /* sc-component-id: sc-bdVaJa */ .sc-bdVaJa {} .jTcPxY{height:100%;overflow:auto;} /* sc-component-id: sc-bwzfXH */ .sc-bwzfXH {} .iYgZUj{color:#BBB;margin:10px 0 20px 0;} /* sc-component-id: sc-htpNat */ .sc-htpNat {} .knYzzU{position:relative;padding:10px 0 10px 0;width:calc(100% - 10px);height:50px;-webkit-transition:-webkit-transform 0.6s;-webkit-transition:transform 0.6s;transition:transform 0.6s;} .knYzzU:hover{background-color:#EEE;-webkit-transform:translate(10px,0);-ms-transform:translate(10px,0);transform:translate(10px,0);}.null{position:relative;padding:10px 0 10px 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Life of xhu

Immutable Object in JavaScript - Part 1

Jul 03, 2016  |  JavaScript  |  Immutable

最近因为写了不少JS代码, 并且开始学习Scala, 所以很多时候会忍不住去对比这两门语言, 说老实话, 初学函数式语言真的是有很大的难度, 不过抛开Scala语言繁杂的语法, 函数式语言的通用特性倒是很简单

  1. 函数是First-class Object, 也就是说函数是作为基本对象存在的
  2. 闭包, 也就是能创造出保存上下文的执行环境, 这个一般都是通过函数来实现
  3. Immutable Object, 不可变对象

这三个特性里, 前两个是JavaScript原生支持的, 但是第三个却没有很好的支持, 这个特性的含义是, 对于一个对象的任何操作, 都应该产生一个新对象, 而不是在原有的对象上进行修改, JS里的Array和Object显然都不符合这样的要求.

其实目前除了函数式编程语言之外, 大多数编程语言里的对象都是Mutable的, 我的一个猜想是, 在计算机硬件还没有像现在这么廉价的时候, 创建对象在代码里是很昂贵的操作, 因为一定需要额外的内存空间. 然而在现在硬件相对比较便宜的情况下, 如果能够获得极大的开发便利和效率提升, 牺牲一点空间也是允许的.

Why Immutable

那么为什么要选择不可变对象呢? 简单一个概念好像的确找不到在JS里的应用场景, 在jQuery的时代, 前端应用好像也的确用不到这么高大上的东西, 直到这几年一系列新的前端框架和理念的出现, 其中有两个非常重要:

  1. 组件化(React, Angular#Directive)
  2. 状态管理(Redux, Angular#ViewModel)

现在一般一个页面都会被分成很多组件, 每个组件都可以进行局部渲染, 避免大规模的重绘, 而这个状态管理就是用来通知组件重新渲染, 在Angular里, dom和数据是动态绑定的, 数据更新通过内置的脏检测机制来告知dom重绘, 这些我们这里不做更深的探讨, 因为我已经忘的差不多了; 而当我们使用Redux管理状态的时候, 我们需要手动告诉组件状态是否改变, 如果状态只是一些基本类型还好说, 直接使用===比较就可以了, 如果是对象呢, 即使改变一个键的值, 因为引用没有改变, 比较的结果仍然是true, 这样是无法得知状态是否更新了的.

所以, 人们自然而然想到使用Immutable Object来表示程序状态了.

事实上Redux也的确是这么做的, 我们可以通过这行代码看出, Redux的确只是简单的比较对象引用是否相等来判断状态是否更新.

How Immutable - Copy Object

既然JS里原生不提供不可变对象, 而不可变对象有那么多好处, 那么我们自己让一个对象不可变不就好了.

产生不可变对象, 最简单的方式就是改变对象的时候不是在原对象上进行操作, 而是复制出一个新对象进行修改, 这样就获得了对一个新对象的引用, 庆幸的是, 在ES6之后, 我们拥有了Object#assign方法, 通过这个方法, 我们就可以很简单的复制一个对象.

Shallow Copy

Object#assign方法的语法很简单, 其实就是和jQuery#extend类似, 这个方法接受一个对象序列, 把第二个以及往后的参数里对象依次merge到第一个参数中, 具体的用法是:

obj1 = { a: 1 };
obj2 = Object.assign({}, obj1);

obj1.a = 2;
console.log(obj2['a']);   // 1
console.log(obj1 === obj2);   // false

只要我们在每次处理对象的时候都进行一次assign操作, 就可以把对象看成是immutable的了, 这个方案看上去很完美了, 但是它真的完美吗? 我们看看这段代码

obj1 = { a: { b: 1 } }
obj2 = Object.assign({}, obj1)

obj1.a.b = 2
console.log(obj2.a)   // { b: 2}
console.log(obj1.a === obj2.a)   // true

最后一个表达式的结果并不是我们预想的false, 为什么呢? 因为assign这个操作是浅复制 Shallow Copy.

什么是浅复制呢? 我们知道, 和Scala不同的是, JS的数据类型并不全是引用类型, 真正的引用类型只有使用typeof操作符结果为object的数据, 基本类型在JS中是值类型的, 而在使用assign方法的时候, 值类型会被复制, 而引用类型, 却不会复制内存, 仍然只是复制之前对象的引用, 这样一来, 复制之后的结果是没法使用相等来判断内部对象是否更新的.

值得一提的是, 函数类型在复制的时候行为和其他类型都有不同, 看下面这段代码:

obj1 = { a: x => x };
obj2 = Object.assign({}, obj1);
console.log(obj1.a === obj2.a);   // true

obj1.a = x => x * x;
console.log(obj1.a(3));   // 9
console.log(obj2.a(3));   // 3
console.log(obj1.a === obj2.a);   // false

obj1.a = x => x;
console.log(obj1.a === obj2.a);   // false

如果把函数当成引用类型, 那么第一个判断式我们会认为函数a只复制了引用, 但是当改变obj1.a的时候, obj2.a并没有跟着改变, 这时的判断相等也变成了false, 看上去很像是值类型, 而当把obj1.a改变回原值的时候, 判断式仍然为false, 这又像是引用类型的结果.

具体的原因就不深究了, 复制JS对象中, 函数类型是一种特殊的存在.

Deep Copy

既然有浅拷贝, 那么必然就有深拷贝 Deep Copy了, 也就是在复制一个对象的时候, 连对象内部引用的内存也复制一遍.

如果是需要复制的内存只有基本类型的话, 那么有一个非常简单的方法:

obj1 = { a: { b: 1 } };
obj2 = JSON.parse(JSON.stringify(obj1));
console.log(obj1.a === obj2.a);   // false

原理就不用说了, 这个方法简单粗暴, 但是缺点也是很明显:

  1. 存在性能问题, stringifyparse都是耗时操作;
  2. 只支持JSON里的数据类型, 不支持函数.

所以这个方案只是一个妥协的结果, 业界已经有了很好的深复制方案, 比如jQuery#extend在扩展对象的时候, 有一个可选的deep参数, 当这个参数为true的时候,就是深复制.

那么我们来看一下具体的代码实现吧:

jQuery.extend = jQuery.fn.extend = function() {
  var options, name, src, copy, copyIsArray, clone,
    target = arguments[ 0 ] || {},
    i = 1,
    length = arguments.length,
    deep = false;

  // 如果第一个参数是bool类型, 那么使用深复制模式
  if ( typeof target === "boolean" ) {
    deep = target;

    // 使用第二个参数作为复制结果
    target = arguments[ i ] || {};
    i++;
  }

  // 如果目标对象不是object类型, 则强制使用一个空对象来放置复制结果
  if ( typeof target !== "object" && !jQuery.isFunction( target ) ) {
    target = {};
  }

  // 如果只有一个参数, 那么把后面的参数扩展到jQuery本身
  if ( i === length ) {
    target = this;
    i--;
  }

  for ( ; i < length; i++ ) {

    // 只复制非空对象
    if ( ( options = arguments[ i ] ) != null ) {

      // 扩展目标对象
      for ( name in options ) {
        src = target[ name ];
        copy = options[ name ];

        // 避免复制过程中出现的环
        if ( target === copy ) {
          continue;
        }

        // 对复制对象中的值进行递归调用, 复制下面所有的对象
        if ( deep && copy && ( jQuery.isPlainObject( copy ) ||
          ( copyIsArray = jQuery.isArray( copy ) ) ) ) {

          if ( copyIsArray ) {
            copyIsArray = false;
            clone = src && jQuery.isArray( src ) ? src : [];

          } else {
            clone = src && jQuery.isPlainObject( src ) ? src : {};
          }

          // 如果是引用类型, 就递归的复制内部的内容
          target[ name ] = jQuery.extend( deep, clone, copy );

        } else if ( copy !== undefined ) {
          target[ name ] = copy;
        }
      }
    }
  }

  // 返回的结果是目标对象
  return target;
};

这段代码主要就是用递归的方法来把引用类型也做了一遍复制. 而且也检测了复制时是否有环的出现, 实际使用的时候还是相当好用的.

那么还有没有更好用的方法呢? 答案是肯定的, 那就是大名鼎鼎的React全家桶里的Immutable.js了, 这个库让产生和操作不可变对象变得非常简单, 具体的用法, 就请看 Part 2 吧~

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