How to understand this async/await exercise?

Question

I am doing some exercises on async/await and I am completely blank on this one:

The ​​opA​ function must be called before ​opB​, and ​opB​ must be called before ​opC​. Call functions such a way that ​C​ then ​B​ then ​A​ is printed out.

const print = (err, contents) => {
  if (err) console.error(err)
  else console.log(contents )
}

const opA = (cb) => {
  setTimeout(() => {
    cb(null, 'A')
  }, 500)
}

const opB = (cb) => {
  setTimeout(() => {
    cb(null, 'B')
  }, 250)
}

const opC = (cb) => {
  setTimeout(() => {
    cb(null, 'C')
  }, 125)
}

My guess is there is a typo in the question, so I should just have the functions print out A B C and not C B A?

My attempt is this:

(async function () {
  await print(opA());
  await print(opB());
  await print(opC());
}());

but I get

    cb(null, 'C')
    ^

TypeError: cb is not a function

Question

I have literally no idea how to solve this one, and don't understand the usage of the print function.

Any help on how to get me going will be much appreciated =)

Answer 1

You said this is an async/await exercise but the code you show is the complete antithesis of async/await - it uses the callback paradigm.

To make your code print C B A you would need to pass callbacks to the opX functions in such an order to make them print their results:

const print = (err, contents) => {
  if (err) console.error(err)
  else console.log(contents )
}

const opA = (cb) => {
  setTimeout(() => {
    cb(null, 'A')
  }, 500)
}

const opB = (cb) => {
  setTimeout(() => {
    cb(null, 'B')
  }, 250)
}

const opC = (cb) => {
  setTimeout(() => {
    cb(null, 'C')
  }, 125)
}

opA(print);
opB(print);
opC(print);