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Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Repair Windows errors before they cause bigger problemsFix Now →A TypeScript discriminated union is a union of object types that share a property—often called a discriminant or tag—whose literal value identifies each variant. Check that property with an if statement or switch, and TypeScript narrows the value to the matching type so you can safely use its variant-specific fields.
How a discriminated union works
Each member of the union describes one valid alternative. The shared tag has a different literal value in each member, so checking it tells TypeScript which alternative is present. The tag’s name is up to you: kind, state, and type are all common choices.
type NetworkState =
| { state: "loading" }
| { state: "failed"; code: number }
| { state: "success"; response: { title: string; duration: number } };
function describe(state: NetworkState): string {
switch (state.state) {
case "loading":
return "Loading";
case "failed":
return `Failed with code ${state.code}`;
case "success":
return `Loaded ${state.response.title}`;
}
}In the failed branch, state is narrowed to the member with a numeric code. In the success branch, it is narrowed to the member with a response. The TypeScript Handbook describes the pattern as a union whose members contain a common property with literal types; checking that property lets the compiler narrow out incompatible members. See the TypeScript Handbook: Narrowing.
Why use distinct variants instead of optional fields?
A broad object with an unconstrained tag and several optional properties can describe combinations that are not actually valid. For example, it may allow a loading state to carry a success response, or a failed state with no error code. A discriminated union ties each tag value to the fields that belong with it.
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| Design | What the type expresses | What happens when you check the tag |
|---|---|---|
| One broad object with optional variant fields | Fields may be absent or present in combinations the application does not intend. | The relationship between a tag and its fields may not be clear to TypeScript, so extra checks or assertions can be needed. |
| Union of tagged object variants | Each tag value belongs to a specific object shape. | TypeScript narrows to the matching shape, making its fields available in that branch. |
This is useful when the alternatives are genuinely distinct—for example, request states, success-or-error results, application actions, or protocol messages. The Handbook also points to network communication and state-management mutations as use cases.
Make a switch exhaustive with never
A switch handles the alternatives that exist today, but a later change might add another member. To make that omission a type error, assign the value remaining in the default branch to never:
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function describe(state: NetworkState): string {
switch (state.state) {
case "loading":
return "Loading";
case "failed":
return `Failed with code ${state.code}`;
case "success":
return `Loaded ${state.response.title}`;
default: {
const exhaustive: never = state;
return exhaustive;
}
}
}If you add another state to NetworkState but do not add a matching case, TypeScript can no longer assign the remaining member to never, and reports an error at the exhaustiveness check. This makes the check valuable for state machines, event handlers, and message processors where every new alternative should prompt a deliberate decision.
The Handbook also notes that, with strictNullChecks and an explicit return type, a missing branch can lead to a missing-return error. The never assignment is a direct, visible way to mark the switch as exhaustive. See the Handbook’s exhaustiveness-check example.
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Can you destructure the tag and payload?
Yes, in the cases covered by TypeScript 4.6’s control-flow analysis. A check on a destructured tag can narrow a correlated destructured field when the destructuring uses const or the relevant parameters are never assigned:
type Action =
| { kind: "number"; payload: number }
| { kind: "text"; payload: string };
function handle(action: Action) {
const { kind, payload } = action;
if (kind === "number") {
payload.toFixed();
} else {
payload.toUpperCase();
}
}Here, checking kind narrows payload to a number or string in the corresponding branch. This correlation should not be assumed for destructured variables that are reassigned. The rule and examples are documented in the TypeScript 4.6 release notes.
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How TypeScript’s support evolved
The core tagged-union narrowing behavior was documented in TypeScript 2.0. TypeScript 3.2 broadened which common properties may act as discriminants: the release notes describe eligible properties as those containing a singleton type such as a literal, null, or undefined, without generics. TypeScript 4.6 added the documented destructuring behavior above. These release notes describe when the capabilities were introduced; they are not a recommendation to target those historical versions. See the TypeScript 2.0 and TypeScript 3.2 release notes.
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