How to write dependencies using async/await in Node.js
Question
I have a scenario in Node.js application
Scenario
-
p1 to p5are promises (In actual code they call database or web services) -
p1, p2can be created instantly (from data in request parameters) -
p3, p4depend onp1for data -
p5depends onp2for data
I want to ensure no promise is waiting un-necessarily.
That means p3, p4 are created as soon as p1 resolves
And similarly p5 is created as soon as p2 resolves
Have created a JSBin for this: https://jsbin.com/ruyozes/edit?js,console
Question :
- Can this code be made cleaner?
- OR Use more of async/await and less of Promise syntax?
- Is there any equivalent of
async.auto(of callback world)
// noprotect const startTime = Date.now(); const log = (...a) => { let timeDiff = Date.now() - startTime; console.log(timeDiff + ': ' + a.join(' ')); }; const createPromise = (tag, time) => { log(tag, 'created', time); return new Promise((resolve) => { setTimeout(resolve, time); }) .then(() => { log(tag, 'fired', time); }); }; /* * Scenario * p1, p2 are independent promises * p3, p4 depend on p1 for data * p5 depends on p2 for data */ async function fn() { let p1 = createPromise('p1', 200); let p2 = createPromise('p2', 50); let p3, p4, p5; p1.then(() => { p3 = createPromise('p3', 1000); p4 = createPromise('p4', 500); }); p2.then(() => { p5 = createPromise('p5', 300); }); await Promise.all([p1, p2]); log('Waiting on p3, p4, p5 now'); await Promise.all([p3, p4, p5]); log('All resolved'); } fn();
<!DOCTYPE html> <html> <head> <meta name="description" content="Dependency tree with async/await"> <meta charset="utf-8"> <meta name="viewport" content="width=device-width"> <title>JS Bin</title> <script src='https://cdnjs.cloudflare.com/ajax/libs/babel-core/5.8.23/browser-polyfill.min.js'></script> </head> <body> </body> </html>
1 answers
solution1
0 2017-04-11 22:07:00
Regarding to the first question, I suggest you do the following:
...
async function fn() {
const p1 = createPromise('p1', 200);
const p2 = createPromise('p2', 50);
const p3 = p1.then(() => createPromise('p3', 1000));
const p4 = p1.then(() => createPromise('p4', 500));
const p5 = p2.then(() => createPromise('p5', 300));
await Promise.all([p3, p4, p5]);
log('All resolved');
}
...
Why? Being more strict, your code can lead to unwanted issues. Imagine that p3 = createPromise('p3', 1000); throws an error, what would be the result of such a situation?
p1.then(() => {
p3 = createPromise('p3', 1000); // what if this line throws an error?
p4 = createPromise('p4', 500); // this line will not be excecuted!
});
If it occurs, you'll not even create p4! In other words, p3, that shouldn't have a direct relationship to p4, ends up influencing p4. It can be avoided using my suggestion.
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