TypeScript Arrays

An array holds an ordered collection of values. A TypeScript number[] allows number elements and the same JavaScript array operations available at runtime.

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

Arrays are objects with mutable indexed elements. TypeScript normally checks the element type but does not automatically enforce bounds at runtime. Use 0-based indexes and remember that array variables can reference the same underlying array object.

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 scores: number[] = [80, 92, 75];
console.log(scores[0]);
console.log(scores.length);
scores.push(88);
console.log(scores.join(", "));

Verified output

80
3
80, 92, 75, 88

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.

Element 0 is 80. length begins at 3. push adds 88 at index 3, so join lists four values in order.

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 scores[99] to raise an exception; a normal out-of-range JavaScript index read yields undefined, even if the static type is number under default settings.

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 array of four city names, append a fifth, and print both the first and last element.

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.

TypeScript Arrays: An array holds an ordered collection of values.

Element 0 is 80. length begins at 3. push adds 88 at index 3, so join lists four values in order.

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