Array Programs

Array programs apply typed collections, map, filter, reduce, and non-mutating operations. A function accepting readonly number[] communicates that it should not modify the caller's collection. Always decide how an empty array should behave.

The reduction adds every item and filter selects positive values. Neither method mutates the original array.

Array.sort() mutates its input and compares strings by default unless a numeric comparator is supplied.

This source was compiled using TypeScript 5.8.3 with strict checking, then executed with Node.js 22.16.0. Its stdout below is the actual captured output. The core logic is runnable without a framework or a remote API; integration-specific examples appear in a separate lab when needed.

TypeScript source

function summarize(values: readonly number[]): { total: number; positives: number } {
  return {
    total: values.reduce((sum, value) => sum + value, 0),
    positives: values.filter(value => value > 0).length,
  };
}
console.log(summarize([4, -2, 7, 0]));

Verified output

{ total: 9, positives: 2 }

Compile: npx tsc --strict --target es2022 --module commonjs array-programs.ts; run: node array-programs.js. For examples importing node: modules, install @types/node as a development dependency; for browser or React projects use the separate lab setup.

The reduction adds every item and filter selects positive values. Neither method mutates the original array.

Before editing the code, predict what the output will be for one changed input. Run it and compare your prediction with the actual result.

Array.sort() mutates its input and compares strings by default unless a numeric comparator is supplied.

Determine whether the issue belongs to the type system, a trust boundary, or actual runtime behavior before choosing a correction.

Return the median of an array using a copied sorted array and handle empty input explicitly.

Record expected success and failure outcomes, run the code, and compare the observed behavior. Keep the exercise compatible with strict type checking.

The reduction adds every item and filter selects positive values. Neither method mutates the original array.

Apply the same reasoning to the practice task, then integrate it into a real app only when the runtime and dependency setup are ready.