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TypeScript·Intermediate·Coding·1 min read

What is a discriminated union?

Short interview answer

It is a union whose members share a property with distinct literal values. Checking that discriminator narrows the value to one member, exposing only the fields valid for that state.

Example

TypeScript
type Result =  | { status: 'ok'; data: string }  | { status: 'error'; message: string };
function handle(r: Result) {  if (r.status === 'ok') return r.data;      // 'message' not available  return r.message;                          // 'data' not available}

Key takeaway

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

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