Group records into map entries.

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

const groups = new Map();
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

Save as snippet.js and run node snippet.js. Uses built-in Node.js and JavaScript APIs. Initialize an array for a missing Map key before appending its values.

✓ Verified

Tested on Ubuntu 24.04.5 LTS · 2026-11-02 18:08:28 UTC · v18.19.1 · nodejs 18.19.1+dfsg-6ubuntu5

Test inputs, results and environment

The exact published code ran in a dedicated Linux test container with prepared inputs. Unprivileged snippet process, external networking disabled, read-only root filesystem, and a fresh temporary working directory. 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) / C.UTF-8
Interpreter invocation: node snippet.js
Container image ID: sha256:32dc2d998158b6554c99b73b533f423696a0f3e31e8cbea9db10b5eee9a2fadd
Code SHA-256: 69a7a2597faf10f66e9bb80b66e0e7d998d189375d58827657d2569f2eec124a
Dependencies: nodejs 18.19.1+dfsg-6ubuntu5

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.