Narrow an unknown JSON value before you trust it

I read a setting back from localStorage and TypeScript just calls it any, so nothing checks it anymore.

Beginner to intermediateRead 6 minDoing: under an hour6 steps

What you'll have at the end

A settings loader that reads the JSON blob back from localStorage, types the parsed value as unknown, and narrows it with a written guard, returning defaults instead of a crash on bad or stale data.

Before you start

You need

A JavaScript object you already save to localStorage as a JSON string under a key you control, and a TypeScript project with strict type checking already turned on.

Not covered

Validating JSON that arrives over a network response rather than your own app's local storage, and reaching for a schema-validation library instead of the hand-written guard this recipe builds.

Leans on

Confront what any actually hides

See an untyped read hide a real bug
Force the compiler to make you check it

Build and prove a real guard

Write a guard that actually checks the shape
Prove the guard rejects what it should

Handle a string that never parses, then ship one function

Catch a string that never parses at all
Combine every check into one loader function

Where to go next

Giving a fetch response a real type instead of any

For when the unknown JSON comes back from a network call instead of your own app's storage, a different source with its own failure shapes.

Model an API response so success and failure can't be confused

For when the value you're narrowing already splits into a success or failure case, rather than one settings shape to accept or reject.

Checked 22 Aug 2026

Part of the TypeScript cookbook