Tukukkk

I am trying to figure out how to resolve an arbitrary number of promises one after another within a for-loop.

I have a function called scrapePage(num) that takes in a number and makes an HTTP call to a certain web-page.
I have tried out a number of approaches to this problem.

1) Use an array of promises (that resolves them in random order):

var promises = ;

   for (var i = 0; i < 4; i++) {

   promises.push(scrapPage(i));

}

$q.all(promises).then(doMoreThings);

2) Use a chain of calls (that is useless for an arbitrary number of calls):

var chain = $q.when();

chain.then(function() {

    return scrapPage(0);

}).then(function() {

    return scrapPage(1);

}).then(function() {

    return scrapPage(2);

}).then(function() {

    return scrapPage(3);

}).then(doMoreThings);

I was playing around with the chain function, trying to get it to work. For testing purposes, I made scrapePage print out "page #n" every time it resolves.

Here's what I have right now:

var chain = $q.when();

for (var i = 0; i < 4; i++) {

    console.log("i: " + i);

    chain = chain.then(function() {

        console.log("ni: " + i);

        return scrapPage(i);

    });                 

}    

I can point out two major problems it has.

First, its output is absolutely weird.

i: 0

i: 1

i: 2

i: 3



i: 4

page #4



i: 4

page #4



i: 4

page #4



i: 4

page #4

Second, I don't know how to make it doMoreThings after all promises are resolved.

Could anyone tell me what I'm missing?

Try changing var to let.

var chain = $q.when();

for (let i = 0; i < 4; i++) {

    console.log("i: " + i);

    chain = chain.then(function() {

        console.log("ni: " + i);

        return scrapPage(i);

    });                 

}

That would probably solve the first problem.

For the second one, you could add a condition like this:

if (i == 4){

    doMoreThings();

}

else {

    return scrapPage(i);

}

instead of just return scrapPage(i);