# 链式优化

原文地址：<http://taobaofed.org/blog/2017/03/16/javascript-functional-programing/>

以前有听说过回调地狱，知道一层又一层的嵌套回调函数是很麻烦的事，会遇到一些作用域的问题，具体现在也说不明白了。记得是在《做质控报告》批量操作的功能。有很多报告，每一份报告又分章节，每一章节里的内容都是可以刷新替换的。比如表格和图片。在这里用到过回调函数的3级嵌套，功能还是实现了，就是代码的不好看，逻辑还可以更清晰得。

现在用Promise多重回调时，不用嵌套了，直接接着后面写。如同回调函数的链式优化一样。当我们要在一个函数里拿到某个值去应用到另一个函数时，我们来回调。

* 多重嵌套的例子 ，用于加减法的函数
* ```javascript
  function sum(a , b){
      return a+b ;
  }

  function sub(a,b){
      return a- b;
  }
  ```
* 当计算6+7+8+10-5+3-4+5:
* ```javascript
  sum(sub(sum(sub(sum(sum(sum(6,7),8),10),5),3),4),5)
  ```

这样太不方便查看，对后续的修改也麻烦。

使用链式优化，lodash也是这样的原理。

```javascript
const unit={
    chain(a){
        this._tem = a;
        return this;
    },
    sum(b){
     this._tem +=b ;
     return this ;
    },
    sub(c){
        this._tem +=c;
        return this;
    },
    value(){
        return this._temp;
    }
}

unit.chain(6).sum(7).sum(8).sum(10).sub(5).sum(3).sub(4).sum(5);
```


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