js 封装promise

JS 封装 Promise 的方法有很多种，以下是一种常见的封装方法：

function myPromise(fn) {
  let state = 'pending';
  let value = null;
  let callbacks = [];

  this.then = function(onFulfilled) {
    return new myPromise(function(resolve) {
      handle({
        onFulfilled: onFulfilled || null,
        resolve: resolve
      });
    });
  };

  function handle(callback) {
    if (state === 'pending') {
      callbacks.push(callback);
      return;
    }

    if (!callback.onFulfilled) {
      callback.resolve(value);
      return;
    }

    let ret = callback.onFulfilled(value);
    callback.resolve(ret);
  }

  function resolve(newValue) {
    if (newValue && (typeof newValue === 'object' || typeof newValue === 'function')) {
      let then = newValue.then;
      if (typeof then === 'function') {
        then.call(newValue, resolve);
        return;
      }
    }

    state = 'fulfilled';
    value = newValue;
    setTimeout(function() {
      callbacks.forEach(function(callback) {
        handle(callback);
      });
    }, 0);
  }

  fn(resolve);
}

这是一个简单的 Promise 封装，可以通过 `then` 方法链式调用。