Using instanceof

The keyword instanceof is an operator (similar to + and &&) which checks if the given object is an instance of a specified class.

The instanceof operator comes from JavaScript, and TypeScript adds additional type narrowing in blocks that depend on the result of an instanceof operation.

Try hovering over value in the if and else if blocks above. You will notice that TypeScript infers that the type of value is User in the if block, and Admin in the else if block.

If the TypeScript compiler isn't absolutely sure, it won't narrow the type. This can happen for variables declared with let instead of const, or in situations where instanceof is computed outside the if statement.