Executing callbacks in sequential order without using Promises

Question

I am trying to execute following array (avoid callbackHell) of functions in a sequential order implementing function runCallbacksInSequence (I need to implement

Answer 1

Have the .reduce callback be a higher-order function, which, when called, calls the next function in the chain with the callback. At the end, you'll have a function chain that will start by calling the first function, then the second, etc:

function first(cb) {
  console.log('first()');
  cb();
}
function second(cb) {
  console.log('second()');
  cb();
}
function third(cb) {
  console.log('third()');
  cb();
}
function last() {
  console.log('last()');
}

let fns = [first, second, third, last];

function runCallbacksInSequence(fns, cb) {
  const chainedFns = fns.reduceRight((acc, f) => () => f(acc), cb);
  return chainedFns();
}

runCallbacksInSequence(fns);

If you wanted the runCallbacksInSequence to accept another callback to run at the end of all of them, then:

function first(cb) {
  console.log('first()');
  cb();
}
function second(cb) {
  console.log('second()');
  cb();
}
function third(cb) {
  console.log('third()');
  cb();
}
function last(cb) {
  console.log('last()');
  cb();
}

let fns = [first, second, third, last];

function runCallbacksInSequence(fns, cb) {
  const chainedFns = fns.reduceRight((acc, f) => () => f(acc), cb);
  return chainedFns();
}

runCallbacksInSequence(fns, () => console.log('outer call'));