tarini tarini - 2 months ago 19
Javascript Question

Execute native js promise in series

I have to call a promise for some async task for each item in an array but I would like to execute these serially.

Promise.all is useful only to have a new promise that merges a list of promise but it does not invoke them in a sequence.

How can I achieve this using standard promise api without third-party libraries like Q, bluebird....


You chain promises using .then() with a callback that returns another promise. So, let's say you have three functions a, b and c that all return a promise. You can chain them (execute in sequence) like this:

a().then(b).then(c).then(function(result) {
   // all are done here

If you are processing an array and you have a promise-returning function myFunc that you want to call for each item in the array, you can use a standard design pattern for arrays and promises with .reduce() to walk through the array one item at a time like this:

var items = [...];

items.reduce(function(p, item) {
    return p.then(function() {
        return myFunc(item);
}, Promise.resolve());

As it turns out this is really just chaining a bunch of .then() handlers like in the first example, but using the structure of .reduce() to walk the array for you.