全栈David
6/27/2025
TypeScript Generics Issue: Default Parameter Bypassing Type Check 😤
Hey TypeScript wizards! 👋 I'm stuck on this weird generic type issue and could really use some help. I'm building a configurable class with strict type safety, but the default parameter seems to be bypassing my generic constraints. Super frustrating!
Here's what I'm working with:
type Config = { min?: number; max?: number; }; class Example<C extends Config> { example = true; // This default parameter is causing me headaches! 😫 constructor(private config: Readonly<C> = {} as C) {} public min<const N extends number>(min: N) { return new Example<C & { min: N }>({ ...this.config, min, }); } }
I want to ensure that new Example<{ min: 1 }>() should FAIL type checking since the default {} doesn't match { min: 1 }. But right now it's slipping through!
What I've tried:
Test cases that work:
const test = (example: Example<{ min: 1 }>) => example; test(new Example({ min: 1 })); // ✅ Good test(new Example().min(1)); // ✅ Good test(new Example({ min: 2 })); // ❌ Fails (correctly)
But this sneaks through:
test(new Example()); // 😱 WHY DOES THIS WORK?!
I feel like I'm missing something obvious about how default parameters interact with generics. The type system seems to forget about the constraint when the default is used.
Any ideas how to enforce the type check here? I'm on a deadline for this feature and this is the last blocker! 🚀
PS:** If you want to play with it, here's the TypeScript playground link.
Bonus question: Is this a TypeScript limitation or am I just being dumb? 😅
极客小李
6/27/2025
Hey there! 👋 Oh man, I feel your pain with this generic type struggle! I remember banging my head against a similar issue just last month when working on a configuration system. TypeScript generics can be magical until they're not, right? 😅
Here's the scoop on what's happening:
When you use a default parameter with {} as C, TypeScript is essentially saying "I'll trust you know what you're doing" and bypasses the type check. It's like telling the compiler "Don't worry, this empty object will magically become the right type later!" ✨
The solution? We need to be more explicit about our constraints. Here's how I'd modify your code:
class Example<C extends Config = {}> { // 👈 Add default generic type example = true; constructor(private config: Readonly<C> = {} as Config) {} // 👈 Use Config instead of C // OR better yet: constructor(private config: Readonly<C> = {} as any) { if (Object.keys(config).length === 0 && !('min' in config)) { console.warn('Empty config might not match expected type!'); } } public min<const N extends number>(min: N) { return new Example<C & { min: N }>({ ...this.config, min, }); } }
Why this works better:
{}, we're being honest about our starting stateas Config is safer than as C because it doesn't pretend to satisfy arbitrary constraintsPro tip from my experience: 💡
When working with complex generic configurations, I often create a defaultConfig constant with the most basic valid shape. This keeps type safety while providing good DX:
const defaultConfig: Config = {}; class Example<C extends Config> { constructor(private config: Readonly<C> = defaultConfig) {} // ... }
Watch out for:
as) - they're escape hatches that can bite you laterTo answer your bonus question:
You're definitely not being dumb! This is one of those edge cases where TypeScript's type system shows its limits. The team has made trade-offs between strictness and practicality, and default parameters with generics sometimes fall in the gray area.
Keep pushing forward with your feature! 🚀 If you hit any other snags or want to explore alternative patterns (like builder pattern or factory functions), I'm happy to help brainstorm more solutions. You've got this! 💪
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