3 ms·

https://github.com/JSMonk/hegel#benefits-over-typescript https://github.com/JSMonk/hegel#benefits-over-typescript

• TheRealPomax 4y ago
  Hmm... const numbers: Array<number> = []; // HegelError: Type "Array<number>" is incompatible with type "Array<number | string>" const numbersOrStrings: Array<string | number> = numbers; The very first example is already an odd choice, given that <number> is type-compatible with <string|number>. Errors should be flagged once we try to call functions on the elements, with calls on number[...] validated based on <number>, and calls on numbersOrStrings[...] validated based on the shared API between <string> and <number> (which is a tiny API surface, but there is one). numbersOrStrings[1] = "Hello, TypeError!"; Why is this not an error based on the incorrect assignment we already had? "HegelError: cannot assign to alias numberOfStrings of numbers, numbers is of type <number>". // HegelError: Property "toFixed" does not exist in "Number | undefined" numbers[1].toFixed(1); Heck even this error is weird: we declared numbers as an array of <number>, not an array of <number|undefined>. If you're going to be that strict, track list sizes and go "HegelError: Property numbers[1] is undefined, which is type-incompatible with <number>".
  • lshevtsov 4y ago
    It's because it is not proven that element with the index 1 exists. Even TypeScript enforces bounds checking these days. As for the first one - as i understand, this will spoil the original array, as it is assigned by reference; so by writing the string into `numbersOrStrings`, you also write it into `numbers`, thus violating the type.