没有承诺只是回调的包装

例子 你可以在node js中使用javascript原生承诺

my cloud 9 code link : https://ide.c9.io/adx2803/native-promises-in-node

/**
* Created by dixit-lab on 20/6/16.
*/

var express = require('express');
var request = require('request');   //Simplified HTTP request client.


var app = express();

function promisify(url) {
    return new Promise(function (resolve, reject) {
    request.get(url, function (error, response, body) {
    if (!error && response.statusCode == 200) {
        resolve(body);
    }
    else {
        reject(error);
    }
    })
    });
}

//get all the albums of a user who have posted post 100
app.get('/listAlbums', function (req, res) {
//get the post with post id 100
promisify('http://jsonplaceholder.typicode.com/posts/100').then(function (result) {
var obj = JSON.parse(result);
return promisify('http://jsonplaceholder.typicode.com/users/' + obj.userId + '/albums')
})
.catch(function (e) {
    console.log(e);
})
.then(function (result) {
    res.end(result);
}
)

})


var server = app.listen(8081, function () {

var host = server.address().address
var port = server.address().port

console.log("Example app listening at http://%s:%s", host, port)

})


//run webservice on browser : http://localhost:8081/listAlbums

承诺不是回调。promise表示异步操作的未来结果。当然，按照你的方式来写，收效甚微。但是如果你按照它们应该被使用的方式来编写它们，你可以用一种类似于同步代码的方式来编写异步代码，并且更容易遵循:

api().then(function(result){
    return api2();
}).then(function(result2){
    return api3();
}).then(function(result3){
     // do work
});

当然，代码并没有减少多少，但是可读性大大提高了。

但这还没有结束。让我们来看看真正的好处:如果您想在任何步骤中检查任何错误，该怎么办?如果是回调就太糟糕了，但如果是承诺，那简直就是小菜一碟:

api().then(function(result){
    return api2();
}).then(function(result2){
    return api3();
}).then(function(result3){
     // do work
}).catch(function(error) {
     //handle any error that may occur before this point
});

几乎和尝试{…} catch块。

更好的是:

api().then(function(result){
    return api2();
}).then(function(result2){
    return api3();
}).then(function(result3){
     // do work
}).catch(function(error) {
     //handle any error that may occur before this point
}).then(function() {
     //do something whether there was an error or not
     //like hiding an spinner if you were performing an AJAX request.
});

更棒的是:如果这3个对api、api2、api3的调用可以同时运行(例如，如果它们是AJAX调用)，但你需要等待这3个调用，那该怎么办?如果没有承诺，你就必须创造出某种反击。有了承诺，使用ES6符号，是另一块蛋糕，非常简洁:

Promise.all([api(), api2(), api3()]).then(function(result) {
    //do work. result is an array contains the values of the three fulfilled promises.
}).catch(function(error) {
    //handle the error. At least one of the promises rejected.
});

希望你现在能从新的角度看待承诺。

承诺概述:

在JS中，我们可以将异步操作(例如数据库调用，AJAX调用)包装在承诺中。通常，我们希望在检索到的数据上运行一些额外的逻辑。JS承诺有处理异步操作结果的处理程序函数。处理程序函数甚至可以在其中包含其他异步操作，这些操作可能依赖于前面的异步操作的值。

一个承诺总是有以下三种状态:

待定:每个承诺的起始状态，既未完成也未拒绝。 完成的:操作成功完成。 rejected:操作失败。

挂起的承诺可以用值来解决/实现或拒绝。然后调用以下将回调作为参数的处理程序方法:

promise .prototype.then():当promise被解析时，该函数的回调参数将被调用。 promise .prototype.catch():当promise被拒绝时，这个函数的回调参数将被调用。

虽然上面的方法技能获得回调参数，但它们远比使用要好 这里只有回调的例子可以说明很多问题:

例子

function createProm(resolveVal, rejectVal) { return new Promise((resolve, reject) => { setTimeout(() => { if (Math.random() > 0.5) { console.log("Resolved"); resolve(resolveVal); } else { console.log("Rejected"); reject(rejectVal); } }, 1000); }); } createProm(1, 2) .then((resVal) => { console.log(resVal); return resVal + 1; }) .then((resVal) => { console.log(resVal); return resVal + 2; }) .catch((rejectVal) => { console.log(rejectVal); return rejectVal + 1; }) .then((resVal) => { console.log(resVal); }) .finally(() => { console.log("Promise done"); });

The createProm function creates a promises which is resolved or rejected based on a random Nr after 1 second If the promise is resolved the first then method is called and the resolved value is passed in as an argument of the callback If the promise is rejected the first catch method is called and the rejected value is passed in as an argument The catch and then methods return promises that's why we can chain them. They wrap any returned value in Promise.resolve and any thrown value (using the throw keyword) in Promise.reject. So any value returned is transformed into a promise and on this promise we can again call a handler function. Promise chains give us more fine tuned control and better overview than nested callbacks. For example the catch method handles all the errors which have occurred before the catch handler.

除了其他答案，ES2015语法与承诺无缝融合，减少了更多的样板代码:

// Sequentially:
api1()
  .then(r1 => api2(r1))
  .then(r2 => api3(r2))
  .then(r3 => {
      // Done
  });

// Parallel:
Promise.all([
    api1(),
    api2(),
    api3()
]).then(([r1, r2, r3]) => {
    // Done
});

是的，promise是异步回调。它们不能做回调所不能做的任何事情，并且在异步中您面临与普通回调相同的问题。

然而，承诺不仅仅是回调。它们是一种非常强大的抽象，允许更清晰、更好的功能代码和更少容易出错的样板文件。

那么主要思想是什么呢?

承诺是表示单个(异步)计算结果的对象。他们只决心达到那个结果一次。这意味着几件事:

promise实现了一个观察者模式:

You don't need to know the callbacks that will use the value before the task completes. Instead of expecting callbacks as arguments to your functions, you can easily return a Promise object The promise will store the value, and you can transparently add a callback whenever you want. It will be called when the result is available. "Transparency" implies that when you have a promise and add a callback to it, it doesn't make a difference to your code whether the result has arrived yet - the API and contracts are the same, simplifying caching/memoisation a lot. You can add multiple callbacks easily

承诺是可链的(如果你愿意，可以是单链的):

If you need to transform the value that a promise represents, you map a transform function over the promise and get back a new promise that represents the transformed result. You cannot synchronously get the value to use it somehow, but you can easily lift the transformation in the promise context. No boilerplate callbacks. If you want to chain two asynchronous tasks, you can use the .then() method. It will take a callback to be called with the first result, and returns a promise for the result of the promise that the callback returns.

听起来复杂吗?接下来是代码示例。

var p1 = api1(); // returning a promise
var p3 = p1.then(function(api1Result) {
    var p2 = api2(); // returning a promise
    return p2; // The result of p2 …
}); // … becomes the result of p3

// So it does not make a difference whether you write
api1().then(function(api1Result) {
    return api2().then(console.log)
})
// or the flattened version
api1().then(function(api1Result) {
    return api2();
}).then(console.log)

扁平化不是神奇的，但你可以很容易地做到。对于嵌套较多的示例，(接近)等价的是

api1().then(api2).then(api3).then(/* do-work-callback */);

如果看到这些方法的代码有助于理解，下面是几个最基本的promise库。

承诺有什么好大惊小怪的?

Promise抽象允许更好的函数组合。例如，在链接之后，all函数为多个并行等待的承诺的组合结果创建一个承诺。

最后但并非最不重要的承诺带有集成的错误处理。计算的结果可能是承诺用一个值来实现，或者用一个理由来拒绝。所有的复合函数都自动处理这个问题，并在承诺链中传播错误，因此您不需要在任何地方显式地关心它——与普通回调实现相反。最后，您可以为所有发生的异常添加一个专用的错误回调。

更不用说把事情变成承诺了。

这是相当微不足道的，实际上与良好的承诺库，看看我如何转换现有的回调API承诺?

除了已经确定的答案，用ES6箭头函数承诺从一个适度发光的小蓝矮星直接变成一个红巨星。它即将坍缩成超新星:

api().then(result => api2()).then(result2 => api3()).then(result3 => console.log(result3))

正如oligofren指出的，如果api调用之间没有参数，你根本不需要匿名包装器函数:

api().then(api2).then(api3).then(r3 => console.log(r3))

最后，如果你想达到超大质量黑洞的水平，你可以期待:

async function callApis() {
    let api1Result = await api();
    let api2Result = await api2(api1Result);
    let api3Result = await api3(api2Result);

    return api3Result;
}