技术控老张
3/14/2025
Hey fellow devs! 👋
Ugh, I'm kinda stuck with this TypeScript generic type check issue, and it's driving me a bit nuts. 😅 So, here's the deal: I'm trying to set up an Example class with generics and default parameters, but I hit a snag. My goal is to create an instance like Example<{ min: 1 }>, and I want TypeScript to scream at me if I mess up the types.
Here's a snippet of what my code looks like:
type Config = { min?: number; max?: number; }; class Example<C extends Config> { example = true; constructor(private config: Readonly<C> = {} as C) {} public min<const N extends number>(min: N) { return new Example<C & { min: N }>({ ...this.config, min, }); } }
Okay, now I've got this test function:
const test = (example: Example<{ min: 1 }>) => example;
It works great for cases that should pass, like:
test(new Example({ min: 1 })); // Yay, this works! test(new Example().min(1)); // Also works! test(new Example({}).min(1)); // And this!
And it fails as expected for cases that shouldn't work:
test(new Example({})); // Fails, as it should! test(new Example({ min: 2 })); // Fails, woohoo! test(new Example().min(2)); // Fails too, nice!
But here's the kicker — when I run test(new Example());, there's no error, and it totally should bomb. The default ({}) isn't supposed to be assignable to { min: 1 }!
So I thought, "Okay, let's change up the test function," like this:
const test = <C extends { min: 1 }>(example: Example<C>) => example; test(new Example()); // Now it errors, but that's not the fix I'm after
But that's just a band-aid, right? What I really want is for const example: Example<{ min: 1 }> = new Example(); to be an instant, no-go, throw-an-error kind of line.
Any tips on how I can get TypeScript to behave here? I'm open to ideas! 🙏
Oh, and here's a link to my playground if you wanna play around with it!
PS: If you help me out, I owe you a virtual coffee or something. 😉
极客小李
3/14/2025
Hey there! 👋 I hear you loud and clear — TypeScript can be quirky sometimes, especially when you're trying to wrangle those generics and type checks. I've spent my fair share of time wrestling with similar issues, so let me lend a hand. 😊
Okay, let's dive into why TypeScript isn't throwing a fit when you instantiate new Example() without any config. The catch here is with the default value you're assigning — {} is considered a valid Config, because it doesn't contradict the type definition. That's why it sneaks through without giving an error.
One way to tackle this is by explicitly ensuring the type constraints are met when you instantiate your class. Instead of relying on the default empty object, you can enforce stricter type checks by refining the constructor logic.
Here's a friendly tweak to your code:
class Example<C extends { min: number }> { example = true; constructor(private config: Readonly<C>) { if (!config.min) { // 🚨 Throw an error if 'min' isn't defined! throw new Error("Config must include 'min' with a specific value."); } } public min<const N extends number>(min: N) { return new Example<C & { min: N }>({ ...this.config, min, }); } }
By removing the default = {} as C, you force anyone using the Example class to provide a config that includes min. Now, when someone tries new Example(), it'll rightfully blow up because min is missing!
💡 Pro Tip: Always ensure your type constraints are explicit when you need strict validation. TypeScript is quite flexible, but that flexibility can work against you when you're expecting it to catch errors.
Another thing I've tripped over is forgetting that TypeScript's type checks are at compile time, not runtime. This means if you rely solely on TypeScript without runtime checks (like above), certain invalid states might sneak through depending on how you set up your types.
If this approach doesn't quite fit what you're looking to achieve, or if you have other quirks up your sleeve you want to tackle, feel free to drop another line! I'm more than happy to keep that virtual coffee train rolling. 🚂☕
Hang in there — TypeScript can be a tough cookie, but once you get the hang of it, you'll be breezing through these types like a pro. Keep coding confidently, and I'm here whenever you need a hand. ✌️