实现简化的 promise ​

function Promise(func) {
  this.fullfilled = false;
  this.rejected = false;
  this.pending = true;
  this.handlers = [];
  this.errorHandlers = [];
  function resolve(...args) {
    this.handlers.forEach(handler => handler(...args));
  }
  function reject(...args) {
    this.errorHandlers.forEach(handler => handler(...args));
  }
  func.call(this, resolve.bind(this), reject.bind(this));
}

Promise.prototype.then = function(func) {
  this.handlers.push(func);
  return this;
};
Promise.prototype.catch = function(func) {
  this.errorHandlers.push(func);
  return this;
};

Promise.race = promises =>
  new Promise((resolve, reject) => {
    promises.forEach(promise => {
      promise.then(resolve, reject);
    });
  });

Promise.all = promises =>
  new Promise((resolve, reject) => {
    let len = promises.length;
    let res = [];
    promises.forEach((p, i) => {
      p.then(r => {
        if (len === 1) {
          resolve(res);
        } else {
          res[i] = r;
        }
        len--;
      }, reject);
    });
  });

// test
const p1 = new Promise(resolve =>
  setTimeout(resolve.bind(null, "resolved"), 3000)
);
p1.then(console.log).then((...args) => console.log("second", ...args));

const p2 = new Promise((resolve, reject) =>
  setTimeout(reject.bind(null, "rejected"), 3000)
);
p2.then(console.log).catch((...args) => console.log("fail", ...args));

function Promise(func) {
  this.fullfilled = false;
  this.rejected = false;
  this.pending = true;
  this.handlers = [];
  this.errorHandlers = [];
  function resolve(...args) {
    this.handlers.forEach(handler => handler(...args));
  }
  function reject(...args) {
    this.errorHandlers.forEach(handler => handler(...args));
  }
  func.call(this, resolve.bind(this), reject.bind(this));
}

Promise.prototype.then = function(func) {
  this.handlers.push(func);
  return this;
};
Promise.prototype.catch = function(func) {
  this.errorHandlers.push(func);
  return this;
};

Promise.race = promises =>
  new Promise((resolve, reject) => {
    promises.forEach(promise => {
      promise.then(resolve, reject);
    });
  });

Promise.all = promises =>
  new Promise((resolve, reject) => {
    let len = promises.length;
    let res = [];
    promises.forEach((p, i) => {
      p.then(r => {
        if (len === 1) {
          resolve(res);
        } else {
          res[i] = r;
        }
        len--;
      }, reject);
    });
  });

// test
const p1 = new Promise(resolve =>
  setTimeout(resolve.bind(null, "resolved"), 3000)
);
p1.then(console.log).then((...args) => console.log("second", ...args));

const p2 = new Promise((resolve, reject) =>
  setTimeout(reject.bind(null, "rejected"), 3000)
);
p2.then(console.log).catch((...args) => console.log("fail", ...args));