JavaScript Reduce an array to chained promises


Example

This design pattern is useful for generating a sequence of asynchronous actions from a list of elements.

There are two variants :

  • the "then" reduction, which builds a chain that continues as long as the chain experiences success.
  • the "catch" reduction, which builds a chain that continues as long as the chain experiences error.

The "then" reduction

This variant of the pattern builds a .then() chain, and might be used for chaining animations, or making a sequence of dependent HTTP requests.

[1, 3, 5, 7, 9].reduce((seq, n) => {
    return seq.then(() => {
        console.log(n);
        return new Promise(res => setTimeout(res, 1000));
    });
}, Promise.resolve()).then(
    () => console.log('done'),
    (e) => console.log(e)
);
// will log 1, 3, 5, 7, 9, 'done' in 1s intervals

Explanation:

  1. We call .reduce() on a source array, and provide Promise.resolve() as an initial value.
  2. Every element reduced will add a .then() to the initial value.
  3. reduce()'s product will be Promise.resolve().then(...).then(...).
  4. We manually append a .then(successHandler, errorHandler) after the reduce, to execute successHandler once all the previous steps have resolved. If any step was to fail, then errorHandler would execute.

Note: The "then" reduction is a sequential counterpart of Promise.all().

The "catch" reduction

This variant of the pattern builds a .catch() chain and might be used for sequentially probing a set of web servers for some mirrored resource until a working server is found.

var working_resource = 5; // one of the values from the source array
[1, 3, 5, 7, 9].reduce((seq, n) => {
    return seq.catch(() => {
        console.log(n);
        if(n === working_resource) { // 5 is working
            return new Promise((resolve, reject) => setTimeout(() => resolve(n), 1000));
        } else { // all other values are not working
            return new Promise((resolve, reject) => setTimeout(reject, 1000));
        }
    });
}, Promise.reject()).then(
    (n) => console.log('success at: ' + n),
    () => console.log('total failure')
);
// will log 1, 3, 5, 'success at 5' at 1s intervals

Explanation:

  1. We call .reduce() on a source array, and provide Promise.reject() as an initial value.
  2. Every element reduced will add a .catch() to the initial value.
  3. reduce()'s product will be Promise.reject().catch(...).catch(...).
  4. We manually append .then(successHandler, errorHandler) after the reduce, to execute successHandler once any of the previous steps has resolved. If all steps were to fail, then errorHandler would execute.

Note: The "catch" reduction is a sequential counterpart of Promise.any() (as implemented in bluebird.js, but not currently in native ECMAScript).