实现一个Promise

promise.js

// 实现一个promise 拥有then方法

function noop() {
}

function resolve(promise) {
  return function(value) {
    if (promise._status !== 'pending') {
      return; // 如果是fullfilled或者rejected，不可逆
    }
    promise._status = 'fullfilled';
    promise._value = value;
    run(promise);
  }
}

function reject(promise) {
  return function(reason) {
    if (promise._status !== 'pending') {
      return; // 如果是fullfilled或者rejected，不可逆
    }
    promise._status = 'rejected';
    promise._value = reason;
    run(promise);
  }
}

function run(promise) {
  if (promise._status === 'pending') {
    return;
  }
  var value = promise._value;
  var callbacks = promise._status === 'fullfilled' ? promise._doneCallbacks : promise._failCallbacks;
  setTimeout(function() {
    for (var i = 0; i < callbacks.length; i++) {
      callbacks[i](value);
    }
  });
  promise._doneCallbacks = [];
  promise._failCallbacks = [];
}

function Promise(resolver) {
  this._status = 'pending'; // pending fullfilled rejected
  this._value = undefined;
  this._doneCallbacks = [];
  this._failCallbacks = [];
  resolver(resolve(this), reject(this));
  
}

function closureCallback(promise, action, flag) {
  return function(value) {
    if (typeof action === 'function') {
      var result;
      try {
        result = action(value);
      } catch(e) {
        reject(promise)(e);
      }
      if (result === promise) {
        var reason = new Error('recycle..is..not..allowed');
        reject(promise)(reason);
      } else if (result instanceof Promise) {
        result.then(function(data) {
          resolve(promise)(data)
        }, function(reason) {
          reject(promise)(reason);
        })
      } else {
        resolve(promise)(result);
      }
    } else {
      flag === 'resolved' ? resolve(promise)(value) : reject(promise)(value);
    }
  }
};

Promise.prototype.then = function(onResolve, onReject) {
  var promise = new Promise(function() {
  });  // 此处仅仅保证promise的传参相同，并无其他作用。
  /**
   * 同一个promise 可能会 var a = new Promise(); 
   * a.then(); 
   * a.then();
   * 同时保证上一个promise能引用本个promise,引入闭包
   */
  this._doneCallbacks.push(closureCallback(promise, onResolve, 'resolved'));
  this._failCallbacks.push(closureCallback(promise, onReject, 'rejected'));
  run(this);
  return promise;
};

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