2019-06-08 js小功能

1. 内置类型判断

Object.prototype.toString.call(date).
substring(8,Object.prototype.toString.call(date).length-1) 

2. 原生实现map

    Array.prototype.map=function(fn){
    var result=[];
    for(var i=0;i

3. 原生实现reduce

4. 原生实现bind

5. 复合函数概念示例

function add(x, y) {
  return x + y;
}

function square(z) {
  return z * z;
}
function double(x) {
  return x * 2;
}
 复合函数
 function compose(mids) {
   return mids.reduce((leftFn, rightFn) => (...args) =>
     rightFn(leftFn(...args))
   );
 }

 const ret = square(add(1,2))
 console.log(ret)

 const middwares = [add, square, double];
 let retFn = compose(middwares)
 console.log(retFn(1,2)) // 18


---------------------------------------------------------------------------


function compose(middlewares) {
    return function() {
      return dispatch(0);
      // 执行第0个
      function dispatch(i) {
        let fn = middlewares[i];
        if (!fn) {
          return Promise.resolve();
        }
        return Promise.resolve(
          fn(function next() {
            // promise完成后，再执行下一个
            return dispatch(i + 1);
          })
        );
      }
    };
  }


//   fn1
//   fn2 2s
//   delay
//   fn3
//   end fn2
//   end fn1


  async function fn1(next) {
    console.log("fn1");
    await next();
    console.log("end fn1");
  }
  
  async function fn2(next) {
    console.log("fn2");
    await delay();
    await next();
    console.log("end fn2");
  }
  
  function fn3(next) {
    console.log("fn3");
  }
  
  function delay() {
    return new Promise((reslove, reject) => {
      setTimeout(() => {
        console.log("delay");
        reslove();
      }, 2000);
    });
  }
  
  const middlewares = [fn1, fn2, fn3];
  const finalFn = compose(middlewares);
  finalFn(); // ?

6. 给对象方法起别名，导致this失效的解决思路（以给document.getElementById起简短的别名为例）

    
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