TypeScript

Patterns
Exhaustive patterns 

 

 In this case, exhaustive means "It covers all cases", e.g. for an enum, so when a new entry is added, it causes an error until this case is considered everywhere. 

 

 Example with never : 

 enum Status {

 Open,

 InProgress,

 Done,

}

function mapStatus(status: Status): string {

 switch (status) {

 case Status.Open:

 return 'open';

 case Status.InProgress:

 return 'in progress';

 case Status.Done:

 return 'done';

 default:

 const _exhaustiveCheck: never = status;

 return _exhaustiveCheck;

 }

}

// Adding an item to Status causes the error:

// TS2322: Type Status is not assignable to type never

// on _exhaustiveChecks in line 16 

 Example with record and return type 

 enum Status {

 Open,

 InProgress,

 Done,

}

const mapping: Record<Status,string> = {

 [Status.Open]: 'open',

 [Status.InProgress]: 'in progress',

 [Status.Done]: 'done'

}

function mapStatus(status: Status): string{

 return mapping[status];

}

// Adding another item in Status causes:

// TS2741: Property [Status.Another] is missing

// on "mapping" in line 7