2.4.JavaScript异步流程控制- Promise核心方法resolvePromise

// 核心方法：处理成功或失败执行的返回值，处理promise2的关系；
function resolvePromise(promise2, x, resolve, reject) {
  // 有可能x===promise2
  if (promise2 === x) {
    return reject(new TypeError('TypeError: Chaining cycle detected for promise #<Promise>'));
  }
  // 如果是第三方的Promise
  let called; // 文档要求，一旦成功，不能调用失败
  if ((x !== null && typeof x === 'object') || typeof x === 'function') {
    // x可能是一个promise
    try {
      // x = {then:function(){}}
      let then = x.then; // 取then方法

      if (typeof then === 'function') {
        then.call(x, function (y) {
          if (!called) { called = true; } else { return; }
          resolvePromise(x, y, resolve, reject); // 递归检查promise
        }, function (r) {
          if (!called) { called = true; } else { return; }
          reject(r);
        });
      } else {
        resolve(x); // 普通值
      }
    } catch (error) {
      if (!called) { called = true; } else { return; }
      reject(error);
    }
  } else {
    // x是普通值，直接返回
    resolve(x);
  }
}