promise的简单实现

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promise

promise的作用是为了避免回调地狱

下面是promise的简单实现

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function Promise(fn){
  if (!fn){ return }
  var me = this;
  var state = "pending";
  var successList = [];
  var failList = [];
  this.then = function(success, fail){
    switch (state) {
      case "pending":
        var failFn = function(){return "null"}
        if(success.toString().match("Promise") != null){
          failFn = function(){return me}
        }
        successList.push(success)
        failList.push(fail || failFn)
        return this;
      case "resolve":
        success();
        return this;
      case "reject":
        fail();
        return this;
    }
  }
  var resolve = function(value){
    state = "resolve";
    for(var i=0; i<successList.length; i++){
      var tem = successList[i](value)
      if(tem instanceof Promise){
        for(i++; i<successList.length; i++){
          tem.then(successList[i], failList[i])
        }
      }else{
        value = tem
      }
    }
  }
  var reject = function(value){
    state = "reject";
    for(var i=0; i<failList.length; i++){
      var tem = failList[i](value)
      if(tem instanceof Promise){
        tem = successList[i](value)
        for(i++; i<failList.length; i++){
          tem.then(successList[i], failList[i])
        }
      }else{
        value = tem
      }
    }
  }
  fn(resolve, reject)
}
var p = function (){
    return new Promise(function(resolve, reject){
        setTimeout(function(){
          reject('p 的结果');
        }, 1000);
    });
}
var p2 = function (input){
    return new Promise(function(resolve, reject){
        setTimeout(function(){
            console.log('p2拿到前面传入的值：' + input)
            reject('p2的结果');
        }, 3000);
    });
}
p()
.then(function(res){console.log('p的结果:' + res); return 'p then方法第一次返回'}, function(res){
  console.log('p reject 的结果:' + res); return 'p reject then方法第一次返回reject'
})
.then(function(res){console.log('p第一次then方法的返回：'+res); return 'p then方法第二次返回'}, function(res){
  console.log('p reject 第一次then方法的返回：'+res); return 'p reject then方法第二次返回reject'
})
.then(p2)
.then(function(res){console.log('p2的结果：' + res)}, function(res){
  console.log('p2 reject 的结果：' + res)
  return "reject"
});