Execute an Array of promises sequentially without using async/await

Question

Lets say I have an array of promises. Each element of my array is a knex.js query builder and is ready to be executed and returns a promise.

How can I run each element of this array sequentially. The array is built dynamically.

let promisesArray = [q1,q2,q3] ;

Each q is not a promise by itself but it will return a promise upon execution.

Answer 1

Here could be a possible option:

let p = Promise.resolve([]);
promisesArray.forEach(q => {
  p = p.then(responses => {
    //based on the nature of each q, to start execution
    //use either q().then() or q.then()
    return q().then(response => {
      //Any further logic can be here.
      console.log(response);
      return responses.concat([response]);
    })
  })
})

p.then(responses => {
  // here you have all of the responses.
})

Answer 2

You can use Array.reduce to reduce the Array into one promise that chains them one after another

let promisesArray = [q1,q2,q3] ;

function runSequentially(promiseArr) {
  return promiseArr.reduce((accum, p) => accum.then(p), Promise.resolve())
}

//Example, this prints.. 1, 2, 3 then "done".
runSequentially([Promise.resolve(1).then(console.log), Promise.resolve(2).then(console.log), Promise.resolve(3).then(console.log)]).then(() => console.log("done"))