Array Methods

Built-in array methods transform, inspect, search, and combine collections. Choose a method whose behavior matches your intent.

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

find returns the first matching element or undefined; includes checks membership; some and every answer boolean questions. Methods such as map and filter produce new arrays, while push and sort modify the existing array. Read the mutability contract of the method you choose.

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 items: number[] = [2, 4, 6, 8];
console.log(items.includes(6));
console.log(items.find(n => n > 5));
console.log(items.some(n => n % 2 !== 0));
console.log(items.slice(1, 3).join(", "));

Verified output

true
6
false
4, 6

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.

includes(6) returns true. find returns the first value greater than 5, which is 6. some returns false because all are even. slice(1,3) makes [4,6] without changing items.

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.

A successful find can return a value, but unsuccessful find returns undefined; do not access its properties without checking.

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.

Use filter to keep numbers at least 5. Use every to test whether they are all even.

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 Methods: Built-in array methods transform, inspect, search, and combine collections.

includes(6) returns true. find returns the first value greater than 5, which is 6. some returns false because all are even. slice(1,3) makes [4,6] without changing items.

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