ts-r
  • Error
  • Reported by the type checker

TS2359

The right-hand side of an 'instanceof' expression must be either of type 'any', a class, function, or other type assignable to the 'Function' interface type, or an object type with a 'Symbol.hasInstance' method.

The right side of `instanceof` isn't a class or constructor. Use a class, or another kind of check.

What it means

x instanceof Y needs Y to be a constructor (or an object with Symbol.hasInstance). Interfaces and type aliases don't exist at runtime, and a plain value can't be used.

Example

index.ts
const shape = { kind: "circle" };

export const ok = {} instanceof shape;
ts-r-check -p tsconfig.json --pretty false ts-r output
index.ts(3,33): error TS2359: The right-hand side of an 'instanceof' expression must be either of type 'any', a class, function, or other type assignable to the 'Function' interface type, or an object type with a 'Symbol.hasInstance' method.

Recorded by node scripts/verify-examples.mjs with ts-r built from commit 6deaf01. The example uses the default tsconfig: strict, target: es2022, module: esnext, moduleResolution: bundler.

How to fix

  • Use a class on the right.
  • To test against an interface, check a discriminant or properties, or write a type guard.