Chain promises with then()

Question

I'm using the when promise library to lift() my node style callbacks and promisify them...

var nodefn = require('when/node');

var one = nodefn.lift(oneFn),
    two = nodefn.lift(twoFn),
    three = nodefn.lift(threeFn);


function oneFn(callback){
    console.log('one');
    callback(null);
}

function twoFn(callback){
    console.log('two');
    callback(null);
}

function threeFn(callback){
    console.log('three');
    callback(null);
}

I want to call the functions in a chain, like this:

one().then(two).then(three).done();

But it gives me an error when calling the second callback function:

Uncaught TypeError: undefined is not a function

The error refers to the callback function within twoFn(callback).

What is the correct way to chain these functions together and execute one after the other?

I haven't used `when` for a while and don't know `when.lift`, but I guess you would need to do: `one().then(function() { return two();}).then(function() {return three();}).done();` — t.niese, Feb 20 '15 at 12:16

score 2 · Accepted Answer · answered Feb 20 '15 at 12:22

2

The problem is that nodefn.lift doesn't know how many parameters the function has (zero), so it just takes the appearing arguments and appends its callback to them. In a then chain every callback receives the result of the previous promise (in your case: undefined), so your twofn will be called with two arguments: undefined and the nodeback.

So if you fix their arities, it should work:

Function.prototype.ofArity = function(n) {
    var fn = this, slice = Array.prototype.slice;
    return function() {
        return fn.apply(null, slice.call(arguments, 0, n));
    };
};
var one = nodefn.lift(oneFn).ofArity(0),
    two = nodefn.lift(twoFn).ofArity(0),
    three = nodefn.lift(threeFn).ofArity(0);

answered Feb 20 '15 at 12:22

Bergi

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You are 100% right, I checked and those were the args passed in (`[undefined, nodeback]`). But do you have any work around or solution? – Drahcir Feb 20 '15 at 12:24
What bugs me a bit that your promises actually resolve with `null` not `undefined`. I wonder whether `when` mixes those up. – Bergi Feb 20 '15 at 12:26
Thanks for the help, please let me know what you think of my answer. – Drahcir Feb 20 '15 at 12:37
1

Really? On the function prototype? – Benjamin Gruenbaum Feb 20 '15 at 13:11
@BenjaminGruenbaum: Sure, I like it :-) Of course it's trivial to change to `function ofArity(fn, n) {…` – Bergi Feb 20 '15 at 14:14

score 0 · Answer 2 · answered Feb 20 '15 at 12:37

0

Based on @Bergi's answer, I wrote this function:

function nodeLift(fn){
    return function(){
        var args = Array.prototype.slice.call(arguments, 0, fn.length - 1),
            lifted = nodefn.lift(fn);

        return lifted.apply(null, args);
    };
}

answered Feb 20 '15 at 12:37

Drahcir

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I think you should use `nodefn.apply` directly, or otherwise put `var lifted = …` above the `return function(){` – Bergi Feb 20 '15 at 12:40
Also note that this function is clever, but it's *too* clever for functions that have optional parameters. Proceed with care :-) – Bergi Feb 20 '15 at 12:43

Chain promises with then()

2 Answers2