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TypeScript·Advanced·Coding·1 min read

How do you enforce exhaustive handling?

Short interview answer

Use a switch on the discriminator and pass the default value to an assertNever function or assign it to never. Adding a new union member then creates a compile error at every non-exhaustive branch.

Example

TypeScript
function assertNever(x: never): never {  throw new Error('Unhandled: ' + JSON.stringify(x));}
switch (result.status) {  case 'ok': return result.data;  case 'error': return result.message;  default: return assertNever(result); // compile error if a case is missing}

Key takeaway

Explain the underlying mental model clearly, then support it with a concrete example and its trade-offs.

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