What is type narrowing in TypeScript?
Why Interviewers Ask This
This question tests conceptual clarity. Interviewers want to hear a precise, confident definition before moving to more complex TypeScript topics. It also reveals how well you can explain technical ideas to non-experts.
Answer
Type narrowing is the process of refining a broader type (like a union) to a more specific type within a conditional block. TypeScript uses control flow analysis to track which types are possible at each point in the code. Common narrowing techniques: typeof guard: if (typeof value === "string") { value.toUpperCase(); }. instanceof guard: if (error instanceof TypeError) { ... }. Truthiness narrowing: if (value) { /* value is not null/undefined/falsy */ }. Equality narrowing: if (x === "hello") { /* x is "hello" */ }. in operator: if ("swim" in animal) { animal.swim(); }. Discriminated unions: a common literal property used to differentiate union members. Custom type guards: functions with a value is Type return type. After narrowing, TypeScript knows the exact type and provides full type checking and autocomplete for that specific type's members.
Pro Tip
Demonstrate both theoretical understanding and practical experience. Say what it is, then give an example of how you actually used it in a TypeScript codebase.