Chaining promises in a waterfall

Question

I've been playing around with a few different ways of chaining a collection of functions and can't seem to find one I particularly like. The following is the last one I settled on but am still not keen on it.

Can someone suggest a cleaner and more concise pattern? I don't want to opt for Async.js or a library.

[
  this.connectDatabase.bind(this),
  this.connectServer.bind(this),
  this.listen.bind(this)
].reduce(
  (chain, fn) => {
    let p = new Promise(fn);
    chain.then(p);
    return p;
  },
  Promise.resolve()
);

Ps. any other tips are more than welcomed.

score 12 · Answer 1 · edited May 23 '17 at 12:03

12

Found this solution on stackoverflow on how you can chain promises dynamically:

iterable.reduce((p, fn) => p.then(fn), Promise.resolve())

The complete post is here: https://stackoverflow.com/a/30823708/4052701

edited May 23 '17 at 12:03

Community

1
1

answered Nov 13 '16 at 23:42

TiagoLr

2,782
22
16

Nice and clean. Thanks! – ddibiase Nov 14 '16 at 02:21
Hmmm, just tried it with my implementation, looks like it was simplified because it moved the promise generation into the individual functions. Wondering if there's something that avoids that entirely. For now this will do though =) – ddibiase Nov 14 '16 at 02:29
This is beautiful :) – robinmitra May 16 '17 at 19:17

Niels Steenbeek · Answer 2 · 2017-05-29T11:15:30.410

2

What about ES7 async/await? Strange/old bind(this) in your code, but kept to not confuse with your example.

async function x() {
    try {
        await this.connectDatabase.bind(this);
        await this.connectServer.bind(this);
        await this.listen.bind(this);
    } catch(e) {
        throw e;
    }
}

or more generic

async function () {
    for (let item of yourArray) {
        try {
            await item.bind(this); //strange bind of your code.
        } catch(e) {
            throw e;
        }
    }
}

edited May 29 '17 at 11:15

answered Nov 14 '16 at 16:10

Niels Steenbeek

4,692
2
41
50

Yeah I'd much prefer to do it this way. Haven't touched ES7 yet, scared to upgrade our codebase. The bind is in there because I haven't learned a new/better pattern for referencing class methods in my class. In this case connectDatabase is a method inside my class that then uses references to additional methods in the class. If I don't bind the method call to this, it loses it's context. Suggestions? – ddibiase Nov 14 '16 at 17:57
Can you please share some working sandbox? What if i do not want to listen if connectServer fails? – codeofnode May 29 '17 at 10:10
@codeofnode I updated my answer by adding try-catch. Your last question can be done by surrounding connectServer method by try-catch: try {await this.connectServer.bind(this);}catch(e){//don't throw to continue}. If you don't want to wait for connectServer, you can remove the await. – Niels Steenbeek May 29 '17 at 11:15

Chaining promises in a waterfall

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