手写Promise

代码比较短的实现

/**
 * 1. new Promise时，需要传递一个 executor 执行器，执行器立刻执行
 * 2. executor 接受两个参数，分别是 resolve 和 reject
 * 3. promise 只能从 pending 到 rejected, 或者从 pending 到 fulfilled
 * 4. promise 的状态一旦确认，就不会再改变
 * 5. promise 都有 then 方法，then 接收两个参数，分别是 promise 成功的回调 onFulfilled, 
 *      和 promise 失败的回调 onRejected
 * 6. 如果调用 then 时，promise已经成功，则执行 onFulfilled，并将promise的值作为参数传递进去。
 *      如果promise已经失败，那么执行 onRejected, 并将 promise 失败的原因作为参数传递进去。
 *      如果promise的状态是pending，需要将onFulfilled和onRejected函数存放起来，等待状态确定后，再依次将对应的函数执行(发布订阅)
 * 7. then 的参数 onFulfilled 和 onRejected 可以缺省
 * 8. promise 可以then多次，promise 的then 方法返回一个 promise
 * 9. 如果 then 返回的是一个结果，那么就会把这个结果作为参数，传递给下一个then的成功的回调(onFulfilled)
 * 10. 如果 then 中抛出了异常，那么就会把这个异常作为参数，传递给下一个then的失败的回调(onRejected)
 * 11.如果 then 返回的是一个promise，那么会等这个promise执行完，promise如果成功，
 *   就走下一个then的成功，如果失败，就走下一个then的失败
 */
 
const PENDING = 'pending';
const FULFILLED = 'fulfilled';
const REJECTED = 'rejected';
function Promise2(executor) {
  let self = this;
  self.status = PENDING;
  self.onFulfilled = [];
  self.onRejected = [];
 
  function resolve(value) {
    if (self.status === PENDING) {
      self.status = FULFILLED;
      self.value = value;
      self.onFulfilled.forEach(fn => fn());
    }
  }
 
  function reject(reason) {
    if (self.status === PENDING) {
      self.status = REJECTED;
      self.reason = reason;
      self.onRejected.forEach(fn => fn());
  }
 
  try {
    executor(resolve, reject);
  } catch (e) {
    reject(e);
  }
}
 
Promise2.prototype.then = function (onFulfilled, onRejected) {
  onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : value => value;
  onRejected = typeof onRejected === 'function' ? onRejected : reason => { throw reason };
  let self = this;
  let promise2 = new Promise2((resolve, reject) => {
    if (self.status === FULFILLED) {
 
      setTimeout(() => {
        try {
          let x = onFulfilled(self.value);
          resolvePromise(promise2, x, resolve, reject);
        } catch (e) {
          reject(e);
        }
      });
    } else if (self.status === REJECTED) {
      setTimeout(() => {
        try {
          let x = onRejected(self.reason);
          resolvePromise(promise2, x, resolve, reject);
        } catch (e) {
          reject(e);
        }
      });
    } else if (self.status === PENDING) {
      self.onFulfilled.push(() => {
        setTimeout(() => {
          try {
            let x = onFulfilled(self.value);
            resolvePromise(promise2, x, resolve, reject);
          } catch (e) {
            reject(e);
          }
        });
      });
      self.onRejected.push(() => {
        setTimeout(() => {
          try {
            let x = onRejected(self.reason);
            resolvePromise(promise2, x, resolve, reject);
          } catch (e) {
            reject(e);
          }
        });
      });
    }
  });
  return promise2;
}
 
function resolvePromise(promise2, x, resolve, reject) {
  let self = this;
  if (promise2 === x) {
    reject(new TypeError('Chaining cycle'));
  }
  if (x && typeof x === 'object' || typeof x === 'function') {
    let used; //PromiseA+2.3.3.3.3 只能调用一次
    try {
      let then = x.then;
      if (typeof then === 'function') {
        then.call(x, (y) => {
          if (used) return;
          used = true;
          resolvePromise(promise2, y, resolve, reject);
        }, (r) => {
          if (used) return;
          used = true;
          reject(r);
        });
 
      }else{
        if (used) return;
        used = true;
        resolve(x);
      }
    } catch (e) {
      if (used) return;
      used = true;
      reject(e);
    }
  } else {
    resolve(x);
  }
}
 
 
 
 
 
var p1 = new Promise2((r, j) => {
  console.log('a');
  setTimeout(()=> {
    r(1)
  }, 1000)
})
console.log(p1)
var p2 = p1.then(v => {
  console.log('b')
  console.log(v+1)
  return v+1
})
console.log(p2)
var p3 = p2.then(v => {
  console.log('c')
  console.log(v+1)
})
console.log(p3)

在控制台 查看 p1, p2, p3的值

来源

-Promise的源码实现（完美符合Promise/A+规范）