How to return the result of a non-blocking call?

Question

Node.js

There is an API function that does a simple thing: it tries to open a file and returns its contents if successful, and returns the string "none" if it fails. I know how to write it with promises and async/await. And how to write it without promises , with a regular callback? Before the invention of promises, they somehow wrote something like this ... There must be something like

const api = () => {
    fs.readFile(fileName, (err, content) => {
        if (err) {
            // api должно вернуть строку "none"
        } else {
            // api должно вернуть content
        }
    })
    return    // либо "none", либо content
}

Naturally, what is written will not work , because return will not wait for the non-blocking call to complete and will work immediately . How to fix?

Answer 1

I know how to write it with promises and async/await. And how to write it without promises, with a regular callback? Before the invention of promises, they somehow wrote this ...

api can also have a callback that can be called when the result of the operation is ready

const api = (cb) => {
    fs.readFile(fileName, (err, content) => {
        if (err) {
            cb("none")
        } else {
            cb(content)
        }
    })
    // return не нужен
}