Sequential execution of functions using promises

Question

I want to sequentially execute some asynchronous functions based on an array.

I thought this would do it :

var Promise = require("bluebird");
Promise.cast([1,2,3]).reduce(function(_,i){
    console.log("Launching  " + i);
    return Promise.delay(Math.floor(Math.random()*1000)).then(function(){
        console.log("Executing  " + i);
    });
},0);

but I get

Launching  1
Launching  2
Launching  3
Executing  2
Executing  1
Executing  3

instead of the desired result which would be

Launching  1
Executing  1
Launching  2
Executing  2
Launching  3
Executing  3

How can I ensure wholly sequential execution here ?

Note that I know how to do that without promises, I'm only interested in full promises solutions.

Benjamin Gruenbaum · Accepted Answer · 2014-03-04 15:19:46Z

2

You are using an old version of Bluebird. The behavior of .reduce was changed in v1 to work this way:

Like .map and .filter, .reduce now allows returning promises and thenables from the iteration function.

( from the changelog )

Please update to the most recent version. This will cause the code to run correctly.

answered Mar 4, 2014 at 15:19

Benjamin Gruenbaum

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2 Comments

Denys Séguret Over a year ago

Yes, upgrading Bluebird to 1.0.8 fixed the problem.

Benjamin Gruenbaum Over a year ago

If anyone is reading this and is "stuck" on a < v1 version of Bluebird and can't update - let me know and I'll add a < v1 answer - otherwise, I'd rather not so more people upgrade to modern Bluebird.

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