Question: Handling of optional properties.

[akolarski]

Hey, I love your library !
I read your article and I've been searching for such tool for years!

I have a question though.
Do you have any advice on how to handle optional properties ?
Here is example, lets say I have an interface representing task:

interface Task {
    name: string; // required
    estimate?: number // optional
}

and I'm creating a Type Guard:

const isTask = createTypeGuard<Task>((task) => {
  if (typeof task === "object" && task !== null && hasProperties(task, "name")) {
    const { name } = task;

    if (typeof name === "string") {
      return { name };
    }
  }
  return null;
});

This seems to works (TypeScript does not complain), but I can pass wrong type of estimate like and isTask will be wrong too :

const myTask = {name: "Example task", estimate: true}; // note that estimate is number not boolean
isTask(myTask) // true

I've modified my Type Guard to include the optional parameter estimate like so:

const isTask = createTypeGuard<Task>((task) => {
  if (typeof task === "object" && task !== null && hasProperties(task, "name", "estimate")) { // 1. added it here
    const { name } = task;

    if (typeof name === "string" && (typeof estimate === "undefined" || typeof estimate === "string") { // 2. checked type or undefined
      return { name, estimate };
    }
  }
  return null;
});

This works great, and handles the optional parameters perfectly. The previous example now returns false.

const myTask = {name: "Example task", estimate: true}; // note that estimate is number not boolean
isTask(myTask) // false

The only downside is that I still need to manually manage the optional properties here:

hasProperties(task, "name", "estimate")

If I add new optional parameter to Task - typescript will not report any issues.

Do you have any advice and if it possible at all to on how to handle optional properties more automatically ?

Best regards,
Alex

[akolarski]

I just wrote the question and found a really elegant way to make it work.

instead of writing

const isTask = createTypeGuard<Task>(...)

I did

const isTask = createTypeGuard<Required<Task>>(...)

This makes every property on the Task required and TypeScript demands every property to be checked regardless if it was original required or optional.

Actually I can't see any issue this Required<...> type to not become even part of the library. That way the parser will always force you the check every single property regardless if it is optional or required.

Edit: I've made a pull request that I believe should make the library work with any types of properties