map(), filter(), and reduce()

map transforms each element, filter keeps elements matching a condition, and reduce combines elements into one accumulator result.

What you will learn: the purpose of map(), filter(), and reduce(), the rules TypeScript checks, and what its JavaScript behavior looks like in a runnable example.

The methods can be chained, but each step has a distinct purpose. A reduce callback receives the current accumulator and current element; an explicit initial value avoids ambiguity, especially for empty arrays.

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 prices: number[] = [5, 12, 20];
const large = prices.filter(price => price >= 10);
const doubled = large.map(price => price * 2);
const total = doubled.reduce((sum, price) => sum + price, 0);
console.log(large.join(", "));
console.log(doubled.join(", "));
console.log(total);

Verified output

12, 20
24, 40
64

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 produces [12,20]; map makes [24,40]. reduce starts at 0, adds 24 then 40, and returns 64. None of these three calls mutates prices.

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.

Use map when you need a new value for every element, not for side effects. Supply an initial value for reduce unless you intentionally require a nonempty array.

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.

Filter odd numbers from [1,2,3,4,5], square them, and add the squares. Explain each intermediate array.

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.

map(), filter(), and reduce(): map transforms each element, filter keeps elements matching a condition, and reduce combines elements into one accumulator result.

filter produces [12,20]; map makes [24,40]. reduce starts at 0, adds 24 then 40, and returns 64. None of these three calls mutates prices.

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