Validate an unknown value with a type guard.
Check a field before treating unknown data as a typed object.
interface Person { name: string; }
function isPerson(value: unknown): value is Person {
return typeof value === "object" && value !== null &&
"name" in value && typeof value.name === "string";
}
const data: unknown = JSON.parse('{"name":"Ada"}');
console.log(isPerson(data) ? data.name : "invalid");Expected output EXAMPLE
Ada
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. A type predicate must match the runtime checks; a type assertion alone would not validate JSON.
Tested on Ubuntu 24.04.5 LTS · · TypeScript 5.9.3; Node.js 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. 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
124aa4a573274d12a17f42ea8ac590f4b15d9a53a57ce0bdc90b6c65f8735619- 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
Ada
Exit code: 0. Standard error: empty. Validation: Exact stdout comparison. System-dependent values above are one recorded run.