code duplication in conditional Promise

Question

I am writing a function in Typescript that involves checking a condition, doing some extra processing if the condition is satisfied, and then doing the bulk of the job.

Here's the catch: the extra processing stuff, that runs if the condition is satisfied, returns a promise. So instead of a clean, simple solution like this:

function openDoor(door) {
 if(door.locked) {
 door.unlock();
 }
 door.handle.turn();
 door.handle.push();
}

It turns into some horrible mess, like this:

function openDoor(door) {
 if(door.locked) {
 door.unlock().then(() => {
 door.handle.turn();
 door.handle.push();
 });
 } else {
 door.handle.turn();
 door.handle.push();
 }
}

Why, you ask?

Unlocking the door takes some time, and I can't turn and push the handle until it's done. Without code duplication, I wouldn't have time to unlock the door before pushing it.

So how can I make this better?

In the shortened example above, the duplicated section is quite small, but troublesome nonetheless. I would like to be able to make changes in a single place rather than two, when I update the code. So, how can I avoid code duplication while keeping my Promise?

Answer 1

You could move the door.locked check into the door.unlock() function (or into a new function) which would allow you return a promise for both cases:

function unlockDoor(door) { 
 return door.locked 
 ? door.unlock()
 : Promise.resolve();
 } 
}
function openDoor(door) {
 unlockDoor(door).then(() => {
 door.handle.turn();
 door.handle.push();
 });
}