TypeScript · Senior · Question 54 of 75

How does a type-safe builder accumulate information in its type?

class Query<T extends object = {}> {
  where<K extends string, V>(k: K, v: V): Query<T & Record<K, V>> { /* ... */ }
}

✓Correct answer: Each method returns the builder with a new type parameter that extends the previous one
2It mutates a global type
3It uses any and casts at the end

Explanation

The final .build() can then return a precise type, and invalid call orders can be prevented.

Further reading

TS Handbook: Generics ↗

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