What are generic constraints?

Without a constraint, a generic type parameter could be anything, so the implementation cannot safely access properties. A constraint narrows the set of allowed type arguments. The compiler then permits operations guaranteed by the constraint and rejects callers that do not satisfy it.

function pluck<T, K extends keyof T>(items: T[], key: K): T[K][] {
  return items.map((item) => item[key]);
}

const names = pluck(
  [{ name: "Asha" }, { name: "Ravi" }],
  "name",
);

Common constraint shapes

• T extends { id: string } requires an id property
• K extends keyof T restricts a key to properties of T
• T extends string restricts a type to string-like values
• T extends (...args: never[]) => unknown restricts a callable type
• A default type parameter can provide a fallback when inference fails

Constraints are checked at the call site, not at runtime. They improve the API but do not validate values. If the constraint says an object has an id, a value cast into that type can still be missing it at runtime.