Unexpected behavior in Typescript: variable type remains "unknown" after validation

Here is a code snippet I'm working with:

You can view and interact with the code on the Typescript Playground.

// this class is imported by the validator.ts module
class EWC extends Error {
    constructor(public message: str...

When working with the assertValidString function, there seems to be an issue where the type of the str argument changes to unknown after a certain point, causing errors later in the code. This is puzzling and I'm seeking clarity on this matter. Any insights based on your expertise with Typescript would be greatly appreciated. Thank you for your help!

Apologies for any language shortcomings. Many thanks in advance <3.

typescripttypescript-generics

Answer №1

In simpler terms, when you assign a value to a variable that is initially of type unknown, the type will automatically broaden to unknown.

Even if TypeScript could determine the type, this widening still occurs.

After using str = str.trim(), you will need to make another assertion.

The issue arises from how TypeScript widens types after assigning a value and then narrows it down again. In this situation, the potential type after assignment is unknown | string, but when unknown is part of a union, the final type reverts back to unknown (with any being the only exception in union types).

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