如何在 node.js 中创建自定义异步函数

Question

Ii'm trying to do something like.

 function fun1(){ for(var j=0;j<20;j++) { var n = j; console.log("i= "+n); } } function fun2() { console.log("In Callback"); } fun1(); fun2();

its working fine till now and got output as expected.

But, I want to call function fun1() and fun2() asynchronously it means fun2() call before fun1() , b'coz fun1() will take some time for complete the execution as compare to fun2() .

How can I achieve this, Node.js provide asynchronous function, Is it already defined function can asynchronous only or we can make them according our need.

Answer 1

There are multiple ways to achieve this in JavaScript (not node-specific, but there are modules that make your life easier):

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callbacks

They are somewhat continuations and it is a shame developers were bothered with handling them manually (compilers used to do that themselves). But they work:

 function callee(callback) { setTimeout(function() { callback(null, 'finished!'); }, 2000); } function caller() { document.write('started!'); callee(function(err, result) { document.write(result); }); } caller();

It is common in the node environment to indicate errors with the first parameter of the callback (like callback(new Error("something wrong!")) ).

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promises

As callbacks get ugly when nested (imagine 10-20 of them, you'd be screwed debugging this), the idea of promises came up. You might know them as Futures from java. They are built-in to ES6, and you can use them beforehand in the node environment with npm i promise -- many client-side frameworks (eg jQuery, AngularJS) have their own implementations. Originally there was Q .

 var Promise = require('promise'); function callee() { return new Promise(function(resolve, reject) { setTimeout(function() { resolve('finished!'); }, 1000); }); } function caller() { document.write('started!'); callee().then(function(result) { document.write(result); }); } caller();

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generators

ES6 has generators . You might know them from python .

They provide asynchronity as well, as they yield new values once they are computed.

I recommend reading Learn ES2015 for more information on that.

My personal opinion is to never ever use generators as they interfere heavily with promises and make debugging really hard.

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async/await

ES7 will make life a whole lot easier with async / await . You can basically say that a function will be performed asynchronously and that you want to await a result (once you are in a async function). ES7 async functions is a good start to read on that. It's like

 async function callee() { return (() => { return new Promise((resolve, reject) => { setTimeout(() => resolve('finished!'), 1000); }) })(); } async function caller() { document.write('started!'); document.write(await callee()); } // the global async wrapper (async function() { caller(); })();

Answer 2

I have tried to provide a better version of @Dominik Schreiber's answer

 async function callee() { return new Promise((resolve, reject) => { setTimeout(() => resolve('finished!'), 1000); }) } async function caller() { console.log('started!'); console.log(await callee()); } caller();

Answer 3

You can add a callback to the event que with

 process.nextTick(callback);

Don't do that, it is almost never what you want. JavaScript is single threaded so adding a callback still blocks the call of f2. If you have a function that takes a long time to run run it in a child_process or even better, create a separate micro service for it.