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Comment by mekster | Hacker News Reader

Casting is as bad as using "any". You can make the compiler think anything you throw at it as that type and you can get bitten at runtime.

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Thats not 100% true in TypeScript. TypeScript actually only allows "up-casts" and "down-cast" (but if the type is neither a supertype nor a subtype the compiler will throw an error).

That’s my understanding too. You just can’t cast types wily Nily.

Ya, I often have to write “this as any as T” because I’m using T as a self type.

Nope. This is not how casting works in TypeScript. To get an unsafe cast you have to go via the ‘any’ or ‘unknown’ types.

e.g.

‘foo as unknown as Dog’

There are a few casts that are allowed that I would consider unsafe, particularly around unions of literals or using empty object literals:

  // No error here
  type Animal = 'dog' | 'cat';
  const notAnimal = 'couch' as Animal;

  // No error here either
  type Food = { isSpicy: boolean };
  const empty = {} as Food;

https://www.typescriptlang.org/play/index.html#code/C4TwDgpg...

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