mattigames on Adding type safety to object IDs in TypeScript

Comment on Adding type safety to object IDs in TypeScript parent

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

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.htm...

int_19h 2y

You shouldn't need to write this kind of thing manually for every such type.

ht85 2y

    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.

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