Type Predicates

A type predicate uses a return annotation such as value is string to connect a boolean result with a narrower parameter type.

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

Predicates are particularly useful as reusable checks in array filtering. Under modern TypeScript inference, some straightforward functions may gain inferred predicates too, but an explicit predicate clearly documents the intent.

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 isNumber(value: number | null): value is number {
  return value !== null;
}
const values: Array<number | null> = [4, null, 9, 2];
const numbers = values.filter(isNumber);
console.log(numbers.join(", "));
console.log(numbers.reduce((a, b) => a + b, 0));

Verified output

4, 9, 2
15

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.

filter calls isNumber for 4, null, 9, and 2; only null fails. The resulting array has 4, 9, 2, and reduce adds them to 15.

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 claim value is number while always returning true; that would make the caller trust unchecked nulls.

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 an isString predicate and filter a mixed array into a safely typed string list.

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.

Type Predicates: A type predicate uses a return annotation such as value is string to connect a boolean result with a narrower parameter type.

filter calls isNumber for 4, null, 9, and 2; only null fails. The resulting array has 4, 9, 2, and reduce adds them to 15.

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