Generic Constraints

A generic constraint restricts which types may be used for a type parameter while still retaining the specific chosen type.

What you will learn: the purpose of generic constraints, the rules TypeScript checks, and what its JavaScript behavior looks like in a runnable example.

Use extends to require a capability such as a length property. The function can then safely use value.length. A constraint is not an instruction to convert unsupported arguments into acceptable ones.

Key distinction: TypeScript checks the declared types during development; emitted JavaScript carries out the value-level operation at runtime. A type declaration alone never validates untrusted runtime input.

Read this illustration from the first labeled part to the last. Compare it with the example and identify which parts are type checks and which parts run in JavaScript.

This self-contained example uses fixed values, so it runs without reading input or depending on an external service. Paste it into the TypeScript Playground or save it as main.ts.

TypeScript program

function lengthOf<T extends { length: number }>(value: T): number {
  return value.length;
}
console.log(lengthOf("hello"));
console.log(lengthOf([10, 20, 30]));

Verified output

5
3

Run locally: npx tsc --strict --target es2022 --module commonjs main.ts, then node main.js. For projects with a tsconfig.json, build using that configuration instead.

Both strings and arrays have a numeric length, so they satisfy the structural constraint. The first call returns 5; the second returns 3.

Check each step: identify the input values, follow the JavaScript operation in execution order, and match the final expression with the exact output shown. An annotation can reject invalid source before execution, but does not change the value printed.

Do not expect lengthOf(12) to compile: number does not satisfy the required length property.

How to debug: reproduce the issue in a minimal example. For compiler errors, inspect the inferred and declared types; for a runtime surprise, log the actual values and follow their references or branches.

Create a generic describeNamed<T extends {name:string}>(item:T) and call it with objects containing extra fields.

Practice routine: predict the result on paper, edit one part of the code, compile under --strict, run the JavaScript, and explain whether TypeScript caught an error or JavaScript simply behaved differently.

Generic Constraints: A generic constraint restricts which types may be used for a type parameter while still retaining the specific chosen type.

Both strings and arrays have a numeric length, so they satisfy the structural constraint. The first call returns 5; the second returns 3.

Before continuing, explain one valid use case, one common pitfall, and the separation between compile-time checking and runtime behavior for this feature.