What are type guards in TypeScript

Short answer

Type guards are expressions that narrow the type of a variable within a conditional block, allowing safe access to type-specific members.

Steps

1. Use typeof for primitives: if (typeof value === 'string') { ... }
2. Use instanceof for classes: if (error instanceof HttpError) { ... }
3. Use in for object properties: if ('id' in obj) { ... }
4. Write custom type predicate functions: function isString(x: unknown): x is string { ... }
5. Use discriminated unions with a shared literal property for exhaustive checks.

Tips

• Custom type predicates are powerful for API response validation and domain modeling.
• Combine type guards with early returns to flatten nested conditionals.

Common issues

• instanceof checks fail across realms like iframes or VM contexts.
• Type guards only narrow within the scope where the condition is true; reassignment can invalidate the narrowed type.