typeof Narrowing

The JavaScript typeof operator can narrow primitive unions inside TypeScript branches.

What you will learn: the purpose of typeof narrowing, the rules TypeScript checks, and what its JavaScript behavior looks like in a runnable example.

Common typeof results include "string", "number", "boolean", "bigint", "symbol", "undefined", "function", and "object". Remember that typeof null is "object", so use an explicit null check when examining objects.

Key distinction: TypeScript checks the declared types during development; emitted JavaScript carries out the value-level operation at runtime. A type declaration alone never validates untrusted runtime input.

Read this illustration from the first labeled part to the last. Compare it with the example and identify which parts are type checks and which parts run in JavaScript.

This self-contained example uses fixed values, so it runs without reading input or depending on an external service. Paste it into the TypeScript Playground or save it as main.ts.

TypeScript program

function describe(value: string | number | boolean): string {
  if (typeof value === "string") return `text:${value.length}`;
  if (typeof value === "number") return `double:${value * 2}`;
  return value ? "enabled" : "disabled";
}
console.log(describe("hi"));
console.log(describe(6));
console.log(describe(false));

Verified output

text:2
double:12
disabled

Run locally: npx tsc --strict --target es2022 --module commonjs main.ts, then node main.js. For projects with a tsconfig.json, build using that configuration instead.

For hi, typeof returns string and length is 2. For 6, the number branch doubles it to 12. After both tests fail, the remaining type is boolean and false maps to disabled.

Check each step: identify the input values, follow the JavaScript operation in execution order, and match the final expression with the exact output shown. An annotation can reject invalid source before execution, but does not change the value printed.

Do not treat typeof value === "object" as proof value is non-null, because typeof null is also object.

How to debug: reproduce the issue in a minimal example. For compiler errors, inspect the inferred and declared types; for a runtime surprise, log the actual values and follow their references or branches.

Extend the function to accept undefined and add a branch that prints "missing".

Practice routine: predict the result on paper, edit one part of the code, compile under --strict, run the JavaScript, and explain whether TypeScript caught an error or JavaScript simply behaved differently.

typeof Narrowing: The JavaScript typeof operator can narrow primitive unions inside TypeScript branches.

For hi, typeof returns string and length is 2. For 6, the number branch doubles it to 12. After both tests fail, the remaining type is boolean and false maps to disabled.

Before continuing, explain one valid use case, one common pitfall, and the separation between compile-time checking and runtime behavior for this feature.