CODEWITHSUNDEEP
javascriptnode.js
italianknows
italianknows

Reputation: 51

JavaScript / Node Promises : Executing them in sequence

Im a little lost and I think Im hitting "cant see the wood for the trees syndrome".

Im a JS NooB and Im trying to understand how to call a set of JS functions (which return promises) in order. Ive done some reading up and have decided that given Im using Node I should use something like bluebird to manage the promises..

What I cant work out is why this piece of code doesnt work

var Promise = require("bluebird");
// My Promise enabled function from oReily Safari book
function countdown(seconds, timername) {
    return new Promise(function (resolve, reject) {
        console.log('Countdown : Starting Countdown ' + timername);
        for (let i = seconds; i >= 0; i--) {
            setTimeout(function () {
                if (i > 0)
                    console.log(timername + ' ' + i + '...');
                else
                {
                    console.log('countdown '+timername+' now=='+i+' resolving!');
                    resolve(console.log("Countdown : timename="+timername+" ended"));
                }
            }, (seconds - i) * 1000);
        }
    });
}

/*Basic test of promise */
/* when this is run I expected countdown(5, 'Basic : Timer1') to execute and then when **resolved** THEN countdown(5, "Basic Timer2") to execute. 
 *however what I see is both timers executing at the same time.. 
 */

console.log('Basic : Countdown promise test');
countdown(5, 'Basic : Timer1').
        then(function ()
        {
            /*Success */
            console.log("Basic : Ended Successfully");
        }).
        then(countdown(5, "Basic : Timer2")
                );

When I run this I am expecting countdown(5,'timer1') to execute first and then , only when timer1 has finished, will timer2 get executed..

However when I run this I get 

Basic : Countdown promise test
Countdown : Starting Countdown Basic : Timer1
Countdown : Starting Countdown Basic : Timer2
Basic : Timer1 5...
Basic : Timer2 5...
Basic : Timer1 4...
Basic : Timer2 4...
Basic : Timer1 3...
Basic : Timer2 3...
Basic : Timer1 2...
Basic : Timer2 2...
Basic : Timer1 1...
Basic : Timer2 1...
countdown Basic : Timer1 now==0 resolving!
Countdown : timename=Basic : Timer1 ended
countdown Basic : Timer2 now==0 resolving!
Countdown : timename=Basic : Timer2 ended
Basic : Ended Successfully
Done.

Im lost..

Many thanks in advance

Upvotes: 1

Views: 118

Answers (3)

LCB
LCB

Reputation: 1050

You can try this project: https://github.com/LvChengbin/sequence

Sequence.all( [
    () => countdown( 5, 'Basic : Timer1' ),
    () => countdown( 5, 'Basic : Timer2' )
] )

And you can use the second argument of Sequence.all method to specify an interval between every two steps.

You can read its documentation to get more information.

Upvotes: 0

Giorgio Cerruti
Giorgio Cerruti

Reputation: 896

console.log('Basic : Countdown promise test');
countdown(5, 'Basic : Timer1').
         then(function ()
            {
                /*Success */
                console.log("Basic : Ended Successfully");
                return countdown(5, "Basic : Timer2");
            }).
            then(function(){
                 console.log("Finish!");
             });

Upvotes: 1

Joel Harkes
Joel Harkes

Reputation: 11661

The last part of your code has a unintended bug:

    then(countdown(5, "Basic : Timer2") );

This means the result of countdown() is used as callback function. (countdown function is directly executed)

instead use 

then(function(lastResult){ countdown(5, "Basic : Timer2") });

the variable lastResult will have the returned value from the earlier promise in the chain.

Upvotes: 4

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