手写promise对象

回答: 手写Promise对象的代码可以参考以下示例：

function MyPromise(fn) {
  this.PromiseState = "pending";
  this.PromiseResult = undefined;
  this.thenCallback = undefined;
  this.catchCallback = undefined;

  var resolve = (value) => {
    if (this.PromiseState === "pending") {
      this.PromiseState = "fulfilled";
      this.PromiseResult = value;
      if (this.thenCallback) {
        this.thenCallback(value);
      }
    }
  };

  var reject = (errValue) => {
    if (this.PromiseState === "pending") {
      this.PromiseState = "rejected";
      this.PromiseResult = errValue;
      setTimeout(() => {
        if (this.catchCallback) {
          this.catchCallback(errValue);
        } else if (this.thenCallback) {
          this.thenCallback(errValue);
        } else {
          throw "catch is not defined!!!!";
        }
      });
    }
  };

  if (fn) {
    fn(resolve, reject);
  }
}

MyPromise.prototype.then = function (onFulfilled) {
  this.thenCallback = onFulfilled;
  return this;
};

MyPromise.prototype.catch = function (onRejected) {
  this.catchCallback = onRejected;
  return this;
};

new MyPromise((resolve, reject) => {
  setTimeout(() => {
    resolve("成功回调");
    // reject("失败的回调");
  }, 1000);
}).then((value) => {
  console.log(value);
}).catch((reason) => {
  console.log(reason);
});

这段代码实现了一个简单的Promise对象，包含了`resolve`和`reject`方法，以及`then`和`catch`方法。当调用`resolve`方法时，会将Promise的状态设置为成功，并执行`then`回调函数；当调用`reject`方法时，会将Promise的状态设置为失败，并执行`catch`回调函数。在上述示例中，通过`setTimeout`模拟了异步操作，以演示Promise的使用。<span class="em">1</span><span class="em">2</span><span class="em">3</span>
#### 引用[.reference_title]
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