Promisify

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

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

For better understanding, let's see an example.

For instance, we have loadScript(src, callback) from the chapter info:callbacks.

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

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

  document.head.append(script);
}

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

The function loads a script with the given src, and then calls callback(err) in case of an error, or callback(null, script) in case of successful loading. That's a widespread agreement for using callbacks, we saw it before.

Let's promisify it.

We'll make a new function loadScriptPromise(src), that does the same (loads the script), but returns a promise instead of using callbacks.

In other words, we pass it only src (no callback) and get a promise in return, that resolves with script when the load is successful, and rejects with the error otherwise.

Here it is:

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

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

As we can see, the new function is a wrapper around the original loadScript function. It calls it providing its own callback that translates to promise resolve/reject.

Now loadScriptPromise fits well in promise-based code. If we like promises more than callbacks (and soon we'll see more reasons for that), then we will use it instead.

In practice we may need to promisify more than one function, so it makes sense to use a helper.

We'll call it promisify(f): it accepts a to-promisify function f and returns a wrapper function.

function promisify(f) {
  return function (...args) { // return a wrapper-function (*)
    return new Promise((resolve, reject) => {
      function callback(err, result) { // our custom 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(...);

The code may look a bit complex, but it's essentially the same that we wrote above, while promisifying loadScript function.

A call to promisify(f) returns a wrapper around f (*). That wrapper returns a promise and forwards the call to the original f, tracking the result in the custom callback (**).

Here, promisify assumes that the original function expects a callback with exactly two arguments (err, result). That's what we encounter most often. Then our custom callback is in exactly the right format, and promisify works great for such a case.

But what if the original f expects a callback with more arguments callback(err, res1, res2, ...)?

We can improve our helper. Let's make a more advanced version of promisify.

• When called as promisify(f) it should work similar to the version above.
• When called as promisify(f, true), it should return the promise that resolves with the array of callback results. That's exactly for callbacks with many arguments.

// promisify(f, true) to get array of results
function promisify(f, manyArgs = false) {
  return function (...args) {
    return new Promise((resolve, reject) => {
      function *!*callback(err, ...results*/!*) { // our custom callback for f
        if (err) {
          reject(err);
        } else {
          // resolve with all callback results if manyArgs is specified
          *!*resolve(manyArgs ? results : results[0]);*/!*
        }
      }

      args.push(callback);

      f.call(this, ...args);
    });
  };
}

// usage:
f = promisify(f, true);
f(...).then(arrayOfResults => ..., err => ...);

As you can see it's essentially the same as above, but resolve is called with only one or all arguments depending on whether manyArgs is truthy.

For more exotic callback formats, like those without err at all: callback(result), we can promisify such functions manually without using the helper.

There are also modules with a bit more flexible promisification functions, e.g. es6-promisify. In Node.js, there's a built-in util.promisify function for that.

Promisification is a great approach, especially when you use `async/await` (see the next chapter), but not a total replacement for callbacks.

Remember, a promise may have only one result, but a callback may technically be called many times.

So promisification is only meant for functions that call the callback once. Further calls will be ignored.