实现 Promise/A+ 规范

// 三种状态
var status = {
  pending: 'pending',
  fulFilled: 'fulFilled',
  rejected: 'rejected',
}

function Promise(exector) {
  // 当前状态
  this.status = status.pending;

  // fulFilled状态的值
  this.value = undefined;

  // rejected状态的值
  this.reason = null;

  // fulFilled状态的回调函数
  this.onFulfilledHandlers = [];

  // rejected状态的回调函数
  this.onRejectedHandlers = [];

  const self = this;

  // 状态从pending变为fulFilled
  function resolve(value) {
    if (value instanceof Promise) {
      return value.then(resolve, reject);
    }
    if (self.status === status.pending) {
      self.status = status.fulFilled;
      self.value = value;
      setTimeout(function () {
        self.onFulfilledHandlers.forEach(function (h) {
          h(self.value);
        });
      });
    }
  }

  // 状态从pending变为rejected
  function reject(reason) {
    if (self.status === status.pending) {
      self.status = status.rejected;
      self.reason = reason;
      setTimeout(function () {
        self.onRejectedHandlers.forEach(function (h) {
          h(self.reason);
        });
      });
    }
  }

  // exector执行时可能会抛异常
  // 如果不捕获，异常会抛到全局
  try {
    exector(resolve, reject);
  } catch (e) {
    reject(e);
  }
}

// then方法
Promise.prototype.then = function (onFulfilled, onRejected) {
  // 如果onFulfilled不是函数，则转为函数
  if (typeof onFulfilled !== 'function') {
    onFulfilled = function (value) {
      return value;
    }
  }

  // 如果onFulfilled不是函数，则转为函数
  if (typeof onRejected !== 'function') {
    onRejected = function (reason) {
      throw reason;
    }
  }

  const self = this;

  // then返回一个promise
  let promise2 = null;

  // onFulfilled或onRejected的返回值
  let x = undefined;

  // 如果已经是fulFilled状态
  if (this.status === status.fulFilled) {
    promise2 = new Promise(function (resolve, reject) {
      setTimeout(function () {
        try {
          x = onFulfilled(self.value);
          resolvePromise(promise2, x, resolve, reject);
        } catch (e) {
          reject(e);
        }
      });
    });
  }

  // 如果已经是rejected状态
  if (this.status === status.rejected) {
    promise2 = new Promise(function (resolve, reject) {
      setTimeout(function () {
        try {
          x = onRejected(self.reason);
          resolvePromise(promise2, x, resolve, reject);
        } catch (e) {
          reject(e);
        }
      });
    });
  }

  // 如果是pending状态
  if (this.status === status.pending) {
    promise2 = new Promise(function (resolve, reject) {
      self.onFulfilledHandlers.push(function (value) {
        try {
          x = onFulfilled(value);
          resolvePromise(promise2, x, resolve, reject);
        } catch (e) {
          reject(e);
        }
      });
      self.onRejectedHandlers.push(function (reason) {
        try {
          x = onRejected(reason);
          resolvePromise(promise2, x, resolve, reject);
        } catch (e) {
          reject(e);
        }
      });
    });
  }

  return promise2;
}

function resolvePromise(promise2, x, resolve, reject) {
  if (promise2 === x) {
    return reject(new TypeError('circular reference'));
  }

  let then = null;
  let called = false;

  if (x instanceof Promise) {
    if (x.status === status.rejected) {
      reject(x.reason);
    } else {
      x.then(function (y) {
        resolvePromise(promise2, y, resolve, reject);
      }, reject);
    }
  } else if (x != null && ((typeof x === 'object' || typeof x === 'function'))) {
    try {
      then = x.then;
      if (typeof then === 'function') {
        then.call(x, function (y) {
          if (called) return;
          called = true;
          resolvePromise(promise2, y, resolve, reject);
        }, function (r) {
          if (called) return;
          called = true;
          reject(r);
        });
      } else {
        resolve(x);
      }
    } catch (e) {
      if (called) return;
      called = true;
      reject(e);
    }
  } else {
    resolve(x);
  }
}

Promise.deferred = Promise.defer = function () {
  var defer = {};
  defer.promise = new Promise(function (resolve, reject) {
    defer.resolve = resolve;
    defer.reject = reject;
  })
  return defer;
}

module.exports = Promise;

检测通过

promises-aplus-tests Promise.js

实现 ES6 Promise 方法

Promise.prototype.catch = function (reject) {
  this.then(null, reject);
}

Promise.all = function (promises) {
  return new Promise(function (resove, reject) {
    const result = [];
    const len = promises.length;
    const count = 0;
    for (var i = 0; i < len; i++) {
      promises[i].then(function (data) {
        result[i] = data;
        if (++count === len) {
          resolve(result);
        }
      }, function (error) {
        reject(error);
      });
    }
  });
}

Promise.race = function (promises) {
  return Promise(function (resolve, reject) {
    var len = promises.length;
    for (var i=0; i < len; i++) {
      promises[i].then(resolve, reject);
    }
  });
}

Promise.resolve = function (value) {
  if (value instanceof Promise) return value
  return new Promise(function (resolve) {
    resolve(value)
  });
}

Promise.reject = function (value) {
  return new Promise(function (resolve, reject) {
    return reject(value);
  });
}