Nodejs异步通信之Promise

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与回调函数的区别

  • 不用写错误条件if (err) return callback(err)

  • Promise能被作为对象返回并被后期调用

  • 回调

    function successCallback(result) {
console.log("Audio file ready at URL: " + result);
}

function failureCallback(error) {
console.error("Error generating audio file: " + error);
}

createAudioFileAsync(audioSettings, successCallback, failureCallback);

  • promise

    const promise = createAudioFileAsync(audioSettings); 
promise.then(successCallback, failureCallback);

or

createAudioFileAsync(audioSettings).then(successCallback, failureCallback);

状态

Promise有三种状态

  • pending: Initial Case where promise instantiated.
  • fulfilled: Success Case which means promise resolved.
  • rejected: Failure Case which means promise rejected.

方法

Promise有六个方法：

  • Promise.all([promise1, promise2, …]);
  • Promise.race([promise1, promise2, …]);
  • Promise.reject(value);
  • Promise.resolve(value);
  • Promise.catch(onRejection);

  • Promise.then(onFulFillment, onRejection);

    API参考

执行函数

  • Executor Functions are Parameter for Promise Constructor which holds Resolve and Reject Callbacks.
  • It is executed immediately by the Promise implementation which provides the resolve and reject functions.
  • It’s Triggered before the Promise constructor even returns the created object.
  • The Resolve and Reject functions are bound to the promise to fulfill or reject.
  • It’s expected to initiate some asynchronous work and then, once that completes, call either the resolve or reject.

可以被覆盖的方法

  • Promise.prototype.catch();
  • Promise.prototype.then();

错误处理

Promise的两种错误处理方式：

  • then方法的第二个参数onRejection回调

  • catch

    'use strict';
// First Approach
yourPromise.catch(function (error) {
// Your Error Callback
});
// Second Approach
yourPromise.then(undefined, function (error) {
// Your Error Callback
});

错误检测：根据catch或onRejection定义的顺序被触发

'use strict';
Promise.catch(onRejected);
Promise.then(onFulfilled, onRejected);
NODE

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