Array Type Annotations

You can write array types as elementType[] or Array<elementType>. Both forms express the same element-type constraint.

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

Annotations make collection contracts readable and reject incompatible inserted values. They do not make the collection immutable. Union elements are possible, but choose narrow homogeneous types where they represent the real data.

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

const readings: Array<number> = [21, 23, 22];
const labels: string[] = ["low", "high"];
console.log(readings.reduce((sum, n) => sum + n, 0));
console.log(labels.join(" / "));

Verified output

66
low / high

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.

readings is a number array and reduce adds 21+23+22 to get 66. labels is a string array and join produces low / high. Both annotation syntaxes guide compile-time checks.

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 treat Array<number> as a promise of fixed length or valid indexes. Its runtime behavior is ordinary JavaScript Array behavior.

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.

Change labels into Array<string>, add a third label, and try pushing a number to see the type error.

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.

Array Type Annotations: You can write array types as elementType[] or Array<elementType>.

readings is a number array and reduce adds 21+23+22 to get 66. labels is a string array and join produces low / high. Both annotation syntaxes guide compile-time checks.

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