带你读《现代Javascript高级教程》二十七、实现符合Promise/A+规范的Promise（3）

MyPromise.prototype.then = function(onFulfilled, onRejected) {
  var self = this;
  onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : function(value) { return value; };
  onRejected = typeof onRejected === 'function' ? onRejected : function(reason) { throw reason; };
  var newPromise = new MyPromise(function(resolve, reject) {
    function handleFulfilled(value) {
      try {
        var x = onFulfilled(value);
        resolvePromise(newPromise, x, resolve, reject);
      } catch (e) {
        reject(e);
      }
    }
    function handleRejected(reason) {
      try {
        var x = onRejected(reason);
        resolvePromise(newPromise, x, resolve, reject);
      } catch (e) {
        reject(e);
      }
    }
    if (self.state === 'fulfilled') {
      setTimeout(function() {
        handleFulfilled(self.value);
      }, 0);
    } else if (self.state === 'rejected') {
      setTimeout(function() {
        handleRejected(self.reason);
      }, 0);
    } else if (self.state === 'pending') {
      self.onFulfilledCallbacks.push(function(value) {
        setTimeout(function() {
          handleFulfilled(value);
        }, 0);
      });
      self.onRejectedCallbacks.push(function(reason) {
        setTimeout(function() {
          handleRejected(reason);
        }, 0);
      });
    }
  });
  return newPromise;};

function resolvePromise(promise, x, resolve, reject) {
  if (promise === x) {
    reject(new TypeError('Circular reference detected.'));
  }
  if (x && typeof x === 'object' || typeof x === 'function') {
    var called = false;
    try {
      var then = x.then;
      if (typeof then === 'function') {
        then.call(
          x,
          function(y) {
            if (!called) {
              called = true;
              resolvePromise(promise, y, resolve, reject);
            }
          },
          function(r) {
            if (!called) {
              called = true;
              reject(r);
            }
          }
        );
      } else {
        resolve(x);
      }
    } catch (e) {
      if (!called) {
        called = true;
        reject(e);
      }
    }
  } else {
    resolve(x);
  }}