7. Promises

Promise 代表异步操作的最终结果。jQuery 用它自己的方式处理 promises，原生 JavaScript 遵循 Promises/A+ 标准实现了最小 API 来处理 promises。

• 7.1 done, fail, always

  done 会在 promise 解决时调用，fail 会在 promise 拒绝时调用，always 总会调用。

  // jQuery
  $promise.done(doneCallback).fail(failCallback).always(alwaysCallback)
  // Native
  promise.then(doneCallback, failCallback).then(alwaysCallback, alwaysCallback)

• 7.2 when

  when 用于处理多个 promises。当全部 promises 被解决时返回，当任一 promise 被拒绝时拒绝。

  // jQuery
  $.when($promise1, $promise2).done((promise1Result, promise2Result) => {
  });
  // Native
  Promise.all([$promise1, $promise2]).then([promise1Result, promise2Result] => {});

• 7.3 Deferred

  Deferred 是创建 promises 的一种方式。

  // jQuery
  function asyncFunc() {
    const defer = new $.Deferred();
    setTimeout(() => {
      if(true) {
        defer.resolve('some_value_computed_asynchronously');
      } else {
        defer.reject('failed');
      }
    }, 1000);
    return defer.promise();
  }
  // Native
  function asyncFunc() {
    return new Promise((resolve, reject) => {
      setTimeout(() => {
        if (true) {
          resolve('some_value_computed_asynchronously');
        } else {
          reject('failed');
        }
      }, 1000);
    });
  }
  // Deferred way
  function defer() {
    const deferred = {};
    const promise = new Promise((resolve, reject) => {
      deferred.resolve = resolve;
      deferred.reject = reject;
    });
    deferred.promise = () => {
      return promise;
    };
    return deferred;
  }
  function asyncFunc() {
    const defer = defer();
    setTimeout(() => {
      if(true) {
        defer.resolve('some_value_computed_asynchronously');
      } else {
        defer.reject('failed');
      }
    }, 1000);
    return defer.promise();
  }