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

fns.reduceRight((acc, f) => f(acc), cb)

runs

[first, second, third, last].reduceRight((acc, f) => f(acc), second)

which turns into

((acc, f) => f(acc))(
    ((acc, f) => f(acc))(
        ((acc, f) => f(acc))(
             ((acc, f) => f(acc))(
                 second,
                 last
             ),
             third
        ),
        second
    ),
    first
)

(because that's what reduceRight does).

The first thing to run is the innermost call,

 ((acc, f) => f(acc))(
     second,
     last
 )

This becomes

last(second)

which (by the definition of last) is equivalent to

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

This expression ignores second, writes last() to the console, and returns undefined.

This leaves our expression as

((acc, f) => f(acc))(
    ((acc, f) => f(acc))(
        ((acc, f) => f(acc))(
             undefined,
             third
        ),
        second
    ),
    first
)

The next innermost call is

((acc, f) => f(acc))(
     undefined,
     third
)

which turns into

third(undefined)

By the definition of third this is equivalent to

(function (cb) {
    console.log('third()');
    cb();
})(undefined)

which in turn executes

console.log('third()');
undefined();

This writes third() to the console, then throws an exception because undefined is not a function.