如何将现有回调转换为 Node.js 中的承诺？
回调：就 JavaScript 而言，回调是作为函数传递给另一个函数的函数。一旦调用函数的结果准备好，该函数就会执行。简单来说，它处理了 JavaScript 的异步需求。
示例：演示这一点的最佳示例是使用 setTimeout()函数，该函数接受回调并延迟 JavaScript 代码的执行。一旦设置为函数的时间结束，回调函数就会执行。
// Defining a callback function
var callback = ()=>{
console.log("Hello! GeeksforGeeks");
}
// Passing the callback function
// as a parameter
setTimeout(callback, 2000);
// The callback gets executed as
// soon as 2 seconds are over.
// This behavior shows the
// execution of a callback.
输出：
Hello! GeeksforGeeks Promises：它与操作中的回调非常相似。但是，使用 Promises 的好处是提高了代码的可读性，因为它使我们免于回调地狱。
Promise 有四种状态
- Pending：承诺尚未完成。它没有成功也没有失败。
- 兑现：承诺以成功告终。
- Rejected：承诺以错误结束。
- Settled：承诺要么出错，要么成功。
将现有回调转换为 Promise：
// Existing Callback
var callback = function(err, success) {
if(err) {
console.log("Geek is very sad!");
}
else {
console.log("Geek is optimistic, "
+ "thus becomes successful");
}
}
var caller = function(status, callback){
if(status === 'Happy')
callback(null, true);
else {
callback(new Error(), false);
}
}
// Calling the caller method
// Executes the success part
caller('Happy', callback);
// Executes the error part
caller('Sad', callback);
输出：
Geek is optimistic, thus becomes successful
Geek is very sad!要遵循的步骤：
- 定义一个名为error的函数，并在其中插入回调函数的错误代码块。
- 定义一个名为 success 的函数并将成功代码块插入其中。
- 然后通过返回 promise 对象来修改调用者代码。
- 按以下任一方式使用成功和错误方法。
- 请参阅下面的代码片段以更好地理解。
// This snippet briefly shows
// the implementation
var error = function(){
// The error codeblock from
// the existing callback.
console.log("Geek is very sad!");
}
var success = function(){
// The success codeblock from
// the existing callback
console.log("Geek is optimistic, "
+ "thus becomes successful");
}
var caller = function(status) {
return new Promise(function(resolve, reject) {
if(status === 'Happy') {
// Calling the resolve function
// when function returns success
resolve();
}
else {
// Calling the reject function
// when function returns failure
reject();
}
});
};
// Throw success
caller('Happy').then(success).catch(error);
// Throw error
caller('Sad').then(success).catch(error);
输出：
Geek is optimistic, thus becomes successful
Geek is very sad!让我们将以下代码片段更改为 Promise 实现。为了使实现运行，我们需要设置一个在节点服务器上运行的 Web 应用程序。
要理解使用示例，我们需要在系统上设置节点服务器。您可以按照以下步骤设置节点服务器。
按照此处所示的步骤安装节点并设置简单的节点应用程序。
示例：这里的场景是我们有一个学生数组，其中 id 和 name 作为值。我们需要获取具有给定 id 的学生的姓名。我们得到了一个现有的回调，需要将其转换为 Promise。
/* The following code snippet depicts a GET request
to the server made by the client, which requests for
the name of the student with a given student id. Here we
have a student array that stores JSON objects of
student id and name. We are going to create a GET
Request using expressjs in node first. Then we will
write a route to get name of student based on id. */
const express = require('express');
const app = express();
// Students array
var students = [
{
id: 101,
name: "Geek A"
},
{
id: 102,
name: "Geek B"
},
{
id: 103,
name: "Geek C"
},
{
id: 104,
name: "Geek D"
}
];
// Definition of the callback function
const callback = (err, student) => {
if(err) {
return `Student with given id ${err} not found`;
}
else {
return "Here is the student: " + student.name;
}
}
// Passing studentid and callback function as parameter
const findName = (studentId, callbackFunction) => {
let student = students.find(function(studentValue) {
return studentValue.id == studentId;
});
// Student not found
if(typeof student === 'undefined') {
return callbackFunction(studentId, false);
}
else { // Student found
return callbackFunction(null, student);
}
}
const getName = (req, res)=>{
// Sending back the response to the server
res.send(findName(req.params.studentId, callback));
}
app.get('/getName/:studentId', getName);
app.listen(8000, 'localhost', function () {
console.log('Server Listening');
});
输出： 
现在，我们可以将回调函数转换为 Promise。在这里，我们可以再次执行前面的步骤，即失败函数中的错误逻辑和成功函数中的成功逻辑。请参阅下面的代码以获得更清晰的信息。
const express = require('express');
const app = express();
var students = [
{
id: 101,
name: "Geek A"
},
{
id: 102,
name: "Geek B"
},
{
id: 103,
name: "Geek C"
},
{
id: 104,
name: "Geek D"
}
];
// Writing the success logic here
const success = (student) => {
return "Here is the student: " + student.name;
}
// Writing the failure logic here.
const failure = (fail) => {
return `Student with the given id ${fail} was not found`;
}
const findName = (studentId) => {
return new Promise(function (resolve, reject) {
let student = students.find(function (studentValue) {
return studentValue.id == studentId;
});
if (student) {
resolve(student);
}
else {
reject(id);
}
});
}
const getName = async (req, res) => {
let answer = await findName(
req.params.studentId).then(success).catch(failure);
res.send(answer);
}
app.get('/getName/:studentId', getName);
app.listen(8000, 'localhost', function () {
console.log('Server Listening');
});
输出： 