Sequence as Promise

It's zero dependency and lightweight function that allows execute array of functions and promises in sequence and returns Promise

What it do?

Behavior very similar to Promise.all, but all promises or functions executes in sequence. Function executes promises with functions one by one and returns promise withresults array

this code

promise1.then(() => promise2.then(() => promise3.then(callback))).then(done);

equivalent to

const sequence = require('sequence-as-promise');

sequence([promise1, promise2, promise3, callback]).then(done);

How to install

with npm

npm i --save sequence-as-promise

with yarn

yarn add sequence-as-promise

Basic usage

We have array of functions with promises, and we need to execute all that functions in sequence

All these functions accepts two arguments, (prevResult, results) => {}

• prevResult the result of previous function or promise call
• results an array of results from previous functions or promises calls

const sequence = require('sequence-as-promise');
sequence([
    Promise.resolve({status: true}),
    (prevResult/*{status: true}*/, results) => {
        return {moveCircleToMiddle: true};
    },
    (prevResult/*{moveCircleToMiddle: true}*/, results) => {
        return {showGrayCircle: true};
    },
    (prevResult/*{showGrayCircle: true}*/, results) => {
        return {showMicrophone: true};
    }
]).then((results) => console.log('all done', results))

Functions that returns promise

Most standard use case is fetch dependant data one by one

const sequence = require('sequence-as-promise');
sequence([
    fetchUser(32),
    (user) => {
        if (user && user.id === 1) {
            return fetchAdminUrls(user.id);
        }

        return fetchUserUrls(user.id);
    },
    (urls) => {//previous fetch resolved and passed as argument
        return urls.map(makeLink)
    }
]).then((results) => {
    const [user, _, links] = results;
    
    renderHTML(user, links);
});

Handle errors

Any function or promise in sequence can throw an error, so we need handle it

const sequence = require('sequence-as-promise');
sequence([
    fetchUser(32),
    (user) => {
        if (user && user.id === 1) {
            return fetchAdminUrls(user.id); //for instance this fetch throws server error
        }

        return fetchUserUrls(user.id);
    },
    (urls) => { //this will not be executed, because previous promise thorws error
        return urls.map(makeLink); 
    }
]).then(
    (results) => {
        const [user, _, links] = results;
        
        renderHTML(user, links);
    },
    (results) => {
        const error = results.pop(); //last item in results always be an error

        renderError(error);
    }
);

More examples

But, if we need to call all that functions with primitive values between them (why not?).

const sequence = require('sequence-as-promise');
sequence([
    () => {
        return {moveCircleToMiddle: true};
    },
    100,
    (prevResult/*100*/, results) => {
        return {showGrayCircle: prevResult};
    },
    (prevResult/*{showGrayCircle: 100}*/, results) => {
        return {showMicrophone: true};
    },
    500,
    (prev, values) => { // prev == 500
        return {moveCircleToTop: true};
    }
]).then((results) => console.log('all done', results))

Or we have Promises in that array of functions.

const sequence = require('sequence-as-promise');
sequence([
    () => new Promise((resolve, reject) => {
        resolve({moveCircleToMiddle: true});
    }),
    () => {
        return {showGrayCircle: true};
    }
]).then(() => console.log('all done'))