instanceof Narrowing

instanceof checks whether an object inherits from a constructor’s prototype, allowing TypeScript to narrow to that class.

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

Use instanceof for actual class instances or built-in objects such as Date. An ordinary JSON object with matching fields is not automatically an instance of the class. Keep structural shape checks separate from prototype-based checks.

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

class Circle {
  constructor(public radius: number) {}
}
class Square {
  constructor(public side: number) {}
}
function size(shape: Circle | Square): number {
  if (shape instanceof Circle) return shape.radius * 2;
  return shape.side;
}
console.log(size(new Circle(5)));
console.log(size(new Square(7)));

Verified output

10
7

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.

new Circle(5) links the instance to Circle.prototype, so instanceof succeeds and diameter 10 is returned. A Square instance fails that check and returns its side length 7.

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.

Never assume a plain object literal with a radius field passes instanceof Circle; class instance identity is different from structural shape.

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.

Use instanceof Date to narrow Date | string before formatting a calendar value.

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.

instanceof Narrowing: instanceof checks whether an object inherits from a constructor’s prototype, allowing TypeScript to narrow to that class.

new Circle(5) links the instance to Circle.prototype, so instanceof succeeds and diameter 10 is returned. A Square instance fails that check and returns its side length 7.

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