5 ms·

What is the problem with the workaround suggested in the last comment there?

• int_19h 3y ago
  You shouldn't need to write this kind of thing manually for every such type.
  • ht85 3y ago
    function is<T extends string>(value: string, prefix: T): value is `${typeof prefix}_${string}` { return value.startsWith(`${prefix}_`) } You can now do `is(id, 'user')`. If you do that often you probably want to create separate functions, e.g.: function isFactory<T extends string>(prefix: T) { return (value: string) => is(value, prefix) } const isUser = isFactory('user') const isOrder = isFactory('order') Not too bad.
• pcthrowaway 3y ago
  Define a type guard using a "type predicate"[1]. For example: type UserId = `user_${string}`; type GroupId = `group_${string}`; const addUserId = (id: UserId) => { // do something } const processId = (id: string) => { if (id.startsWith('user_') { // type error here: addUserId(id); } else if (otherCondition) // do other things } } Instead you define a function: const isUserId = (some: string): some is UserId => some.startsWith('user_'); Now you can use it as follows: const processId = (id: string) => { if (isUserId(id)) { // no more error: addUserId(id); } else if (otherCondition) // do other things } } [1]: https://www.typescriptlang.org/docs/handbook/2/narrowing.html#using-type-predicates https://www.typescriptlang.org/docs/handbook/2/narrowing.htm...