An example of using Javascript Promises to ensure that a set of asyncronous steps are always performed in the specified order and strictly one after another.

sequencer.js

/*

  An example of using Javascript Promises to ensure that a set of asyncronous
  steps are always performed in the specified order and strictly one after
  another.

*/

// --- some async function that takes some time to complete

function step(param) { // must return a promise

    return new Promise((resolve) => { // using setTimeout here is just an example - your async function might be a web request or a database lookup
        setTimeout(() => {
            console.log(param);
            resolve();
        }, Math.random() * 1000);
    });
}

// --- the main sequence pump - returns a promise for when all steps are completed

function sequence(func, steps) {

    let step = Promise.resolve();

    for (let i = 0; i < steps.length; i++) {
        step = step.then(() => {
            return func(steps[i]);
        });
    }

    return step;
}

// --- the main entry point

let sequenced = true; // try setting this to true and then false to see the difference in results
let params = ['four', 'score', 'and', 'seven', 'years', 'ago'];

if (sequenced) {

    console.log('-- started in sequenced mode --')
    sequence(step, params).then(() => console.log('-- finished --')); // these steps will always be done in order

} else {

    console.log('-- started in unsequenced mode --')

    for (let i = 0; i < params.length; i++) { // these steps will happen in an undetermined order
        step(params[i]);
    }

    console.log('-- finished --')
}

Here is the output with sequenced set to TRUE

-- started in sequenced mode -- four score and seven years ago -- finished --

Here is the output with sequenced set to FALSE

-- started in unsequenced mode -- -- finished -- seven score ago and four years

If you want a full-blown solution to method chaining in the face of asynchronous functions - see my other gist https://gist.github.com/pete-rai/fb197af2b0ec7838216c6728c3ba2468