3 ms·

Its bit sad that startsWith doesn't narrow the type, making this pattern slightly less convenient. The GH issue: https://github.com/microsoft/TypeScript/issues/

• mattigames 3y ago
  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...
• DylanSp 3y ago
  I wish that TS had better type narrowing for the JS standard library, though there's a lot of constraints and design limitations that make it impractical. I ran into a similar issue with the some() method on Array not narrowing types a while back [1]; that issue links to the same sort of issue with filter(), as well as issues where the TS team has discussed what they can and can't do in control flow analysis. [1] https://github.com/microsoft/TypeScript/issues/40844 https://github.com/microsoft/TypeScript/issues/40844
  • EvertBouw 3y ago
    you can improve it a bit with the library ts-reset https://github.com/total-typescript/ts-reset https://github.com/total-typescript/ts-reset
    • presentation 3y ago
      And also can declare your own wrappers to at least achieve it for your own codebase.