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();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 plete the execution as pare 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.
Share Improve this question edited Aug 25, 2015 at 7:02 seenukarthi 8,64410 gold badges50 silver badges73 bronze badges asked Aug 25, 2015 at 6:48 AnkitAnkit 2213 gold badges6 silver badges13 bronze badges 1- Not sure what exactly you are asking, but i think what you are looking is control flow. For that, take a look at npmjs./package/async or even better with promises and generators. gist.github./swarajgiri/16202e32aa4d80d45c62 – Swaraj Giri Commented Aug 25, 2015 at 6:51
3 Answers
Reset to default 10There are multiple ways to achieve this in JavaScript (not node-specific, but there are modules that make your life easier):
callbacks
They are somewhat continuations and it is a shame developers were bothered with handling them manually (pilers 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 mon in the node environment to indicate errors with the first parameter of the callback (like callback(new Error("something wrong!"))).
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 (e.g. 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();generators
ES6 has generators. You might know them from python.
They provide asynchronity as well, as they yield new values once they are puted.
I remend 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.
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();
})();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();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.