Restrictions on classes in Typescript using generics

Question

Restrictions on classes in Typescript using generics

I'm currently working on a straightforward Local Storage utility method. My goal is to retrieve an entry based on a key and a specified type.

Here's the implementation:

get<T>(key: string): T {
  const data = localStorage.getItem(key);
  const object = JSON.parse(data) as T;
  if (!object) {
    throw new Error(`Unable to cast ${JSON.stringify(data)`);
  }
  return object;
}

Unfortunately, the casting as T may not be efficient in all cases. For example, if my Local Storage entry is <'key', 10>, Then:

get<NoMatterTheClass>('key')

will return 10 as a number without throwing any errors.

My question is: How can I ensure that the cast is always successful? Should I use a generic constraint? I've tried using

<T extends (new() => T)>

but it didn't work as expected.

Thanks for any insights you can provide!

typescript typescript-generics

Answer 1

Answer №1

Unfortunately, achieving this is not possible. The use of as T as a type assertion in TypeScript will throw an error if the specified type does not match. However, it's important to note that type checking only occurs during development time and not runtime. This means that JSON.parse(data) as T; does not actually convert JSON.parse(data) into T. Instead, it simply informs the compiler that you are certain JSON.parse(data) conforms to type T when assigned with as T. Therefore, accomplishing this task with TypeScript is not feasible. If needed, JavaScript can be used for such checks.

Answer 2

Unfortunately, achieving this is not possible. The use of as T as a type assertion in TypeScript will throw an error if the specified type does not match. However, it's important to note that type checking only occurs during development time and not runtime. This means that JSON.parse(data) as T; does not actually convert JSON.parse(data) into T. Instead, it simply informs the compiler that you are certain JSON.parse(data) conforms to type T when assigned with as T. Therefore, accomplishing this task with TypeScript is not feasible. If needed, JavaScript can be used for such checks.

Restrictions on classes in Typescript using generics

Answer №1

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