Group records into map entries.

TypeScript: Initialize an array for a missing Map key before appending its values.

const groups = new Map<string, string[]>();
for (const [team, name] of [["A", "Ada"], ["B", "Linus"], ["A", "Grace"]]) {
  const names = groups.get(team) ?? [];
  names.push(name);
  groups.set(team, names);
}
console.log(JSON.stringify([...groups]));

Expected output EXAMPLE

[["A",["Ada","Grace"]],["B",["Linus"]]]

Details

Requires TypeScript and Node.js. Save as snippet.ts, compile with tsc --strict --target ES2022 --module commonjs snippet.ts, then run node snippet.js. Initialize an array for a missing Map key before appending its values.

✓ Verified

Tested on Ubuntu 24.04.5 LTS · 2026-11-02 18:25:04 UTC · TypeScript 5.9.3; Node.js v18.19.1 · bash 5.2.21-2ubuntu4; coreutils 9.4-3ubuntu6.3

Test inputs, results and environment

The exact published code ran in a dedicated Linux test container with prepared inputs. Snippet process uid 65534 with a read-only root filesystem and temporary working files. Networking disabled. Fixtures and output records are isolated from the application database. Testing covers this example and its stated checks, not every input, operating system, or runtime version. The Linux container shares its host's kernel; this is not a native Windows test.

Architecture / host kernel: x86_64 / 6.18.54.1-microsoft-standard-WSL2
Snippet identity / locale: uid=65534, gid=65534 (unprivileged process) / C.UTF-8
Interpreter invocation: tsc --strict --target ES2022 --module commonjs snippet.ts && node snippet.js
Container image ID: sha256:ec91410a8f6ec3e985830b5e3a0560306d36f8c1c5ccd91a6699abe707123fa3
Code SHA-256: f0210ecbe8983bf65434950bb674abcb2666889fc6495e19523766bd88c18e36
Dependencies: bash 5.2.21-2ubuntu4; coreutils 9.4-3ubuntu6.3

Test input setup

These are the prepared inputs used by the test harness. See Details above for the prerequisites and how to run this example.

# No input files required.

Observed standard output

[["A",["Ada","Grace"]],["B",["Linus"]]]

Exit code: 0. Standard error: empty. Validation: Exact stdout comparison. System-dependent values above are one recorded run.