Type | Treat 2021 – Day 3

Orta Therox

Type | Treat Challenge 3

Welcome to the third Type | Treat challenge! These challenges are a series of blog posts which have 2 code challenges in, one for beginners and one for intermediate TypeScript programmers. We’re on day three, which means going over the answers from yesterday and have 2 new challenges.

Yesterday’s Solution

Beginner/Learner Challenge

There is many ways to decide how to type existing data, you could use literal types when you’re sure of the exact formats – or be more liberal and use string when you expect a variety. We opted for literals, but using string is totally cool too.

type UnderripePumpkin = {
    color: "green",
    soundWhenHit: "dull thud"
type RipePumpkin = {
    color: "purple" | "orange" | "blue",
    soundWhenHit: "echo-y"
type OverripePumpkin = {
    color: "green" | "white",
    soundWhenHit: "squishy"

The second part of the challenge used type predicates (or type guards) annotates a function which returns a booleon with narrowing information about the paramters. This means we can tell TypeScript that when the return values to isRipeis true, then the argument pumpkin is of the type RipePumpkin:

- function isRipe(pumpkin: any) {
+ function isRipe(pumpkin: any): pumpkin is RipePumpkin {
       return "soundWhenHit" in pumpkin && pumpkin.soundWhenHit === "echo-y"

Successfully completing this challenge would have no errors.

Our answer.

Intermediate/Advanced Challenge

This challenge was first about understanding different read vs write properties available in both classes and interface/typed objects. Personally, I’ve seen this with document.location a lot where you always get a rich object when you read but can write to that property with a string. We wanted a similar concept, but using punch which for me is generally a ‘throw it all in and see what happens’ style of drink.

class PunchMixer {
  #punch: Punch = {flavour: '', ingredients: []};

  public get punch(): Punch {
      return this.#punch;

  public set punch(punch: Punch | Punch['ingredients'][number]) {
      if (typeof punch === 'string') {
      } else if ('flavour' in punch) {
          this.#punch = punch;
      } else {

This solution uses a mix of private class fields, indexed types and type narrowing to set up a local punch object which is always returned.

The next step was to make this class generic in some form so that a type parameter passed in to the class would dictate what the return value of a vend function was.

- class PunchMixer {
+ class PunchMixer<MixerType> {
+    mixer!: MixerType;

  // ...
+   public vend(): MixerType {
+        return this.mixer;
+    }

We were not too worried about how you passed back the MixerType – our first draft had return {} as MixerType but a private field feels nicer.

Our Answer.

The Challenge

Beginner/Learner Challenge

Figure out how to stop people giving you bad strings for your custom lengths.

Intermediate/Advanced Challenge

Stop losing your literals in Halloween posters

How To Share Your Solution

Once you feel you have completed the challenge, you will need to select the Share button in the playground. This will automatically copy a playground URL to your clipboard.

Then either:

  • Go to Twitter, and create a tweet about the challenge, add the link to your code and mention the @TypeScript Twitter account with the hashtag #TypeOrTreat.
  • Leave us a comment with your feedback on the dev.to post, or in this post.

Best Resources for Additional Help

If you need additional help you can utilize the following:

Happy Typing 🙂


Discussion is closed. Login to edit/delete existing comments.

  • Josh Karges 0

    The proposed solution for the Beginner doesn’t exclude string types like ‘0-12cm’ or ‘0NaNcm’. Is there a way to truly only include positive numbers followed by a unit?

  • Alex Fung 0

    Besides Josh’s observation that the answer cannot reject the “complications”, I notice that req(” 12 cm”) does not fail without any particular handling.
    Moreover, the answer does not fail a bare number req(“12”), which it should.

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