Promisification is a long keyword for a simple transformation. It's the conversion of a function that accepts a callback to a function that returns a promise.

Such transformation are often required in real-life, as many function and libraries are callback-based. But promises are more convenient, so it makes sense to promisify them.

CALLBACK FUNCTION

function loadScript(src, callback) {
    let script = document.createElement('script'); 
    script.src = src; 

    script.onload = () => callback(null, script); 
    script.onerror = () => callback(new Error(`Script error for ${src}`)); 

    document.head.append(script); 
}

//usage
loadScript('path/script.js', (err, script) => {...})

---

Promisify

The new loadScriptPromise(src) function achieves the same result, but it accepts only src (no callback) and returns a promise.

Promise function -

let loadScriptPrommise = function(src) {
    return new Promise((resolve,reject) => {
        loadScript(src, (err, script) => {
            if (err) reject(err)
            else esolve(script); 
        }); 
    })
}

//usage
loadScriptPromise('path/script.js').then(...)

Let's promisify

Now loadScriptPromise fits well in promise-based code. As we can see, it delegates all the work to the original loadScript, providing its own callback that translates to promise resolve/reject.

In practice we'll probably need to promisify many functions, so it makes sense to use a helper. We'll call t pomisify(f): it accepts a to-promisify function f and returns a wrapper function.

That wrapper does the same as in the code above: ;returns a promise an passes the call to original f, tracking the result in a custom callback.

function promisify(f) {
    return function (...args) {
        // return a wrapper function
        return new Promise((resolve, reject) => {
            function callback(err, result) {
                // our cusotm callback for f
                if (err) {
                    reject (err); 
                } else {
                    resolve(result);
                }
            }
            args.push(callback); 
            //append our custom callback
            // to the end of f arguments

            f.call(this, ...args); 
            // call the original function
        }); 
    }; 
}; 

// usage: 
let loadScriptPromise = promisify(loadScript); 
loadScriptPromise(...).then(...);

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