用js写一个promise源码

以下是一个基本的Promise源码实现。它包括Promise对象的构造函数以及then方法和catch方法的实现。该实现仅为示例，不是完整的Promise实现。

function Promise(fn) {
  var state = 'pending';
  var value;
  var callbacks = [];

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

  this.catch = function(onError) {
    return this.then(null, onError);
  };

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

    var cb = state === 'fulfilled' ? callback.onFulfilled : callback.onRejected;
    var next = state === 'fulfilled' ? callback.resolve : callback.reject;

    if (!cb) {
      next(value);
      return;
    }

    try {
      var ret = cb(value);
      next(ret);
    } catch (e) {
      callback.reject(e);
    }
  }

  function resolve(newValue) {
    if (newValue && (typeof newValue === 'object' || typeof newValue === 'function')) {
      var then = newValue.then;
      if (typeof then === 'function') {
        then.call(newValue, resolve, reject);
        return;
      }
    }
    state = 'fulfilled';
    value = newValue;
    setTimeout(function() {
      callbacks.forEach(function(callback) {
        handle(callback);
      });
    }, 0);
  }

  function reject(reason) {
    state = 'rejected';
    value = reason;
    setTimeout(function() {
      callbacks.forEach(function(callback) {
        handle(callback);
      });
    }, 0);
  }

  fn(resolve, reject);
}