TypeScript · Middle · Question 48 of 75

How do you make a generic parameter optional with a default value?

✓Correct answer: type Box<T = string> = { value: T }
2type Box<T?> = { value: T }
3type Box<T | string> = { value: T }

Explanation

Type parameters can have defaults: Box without an argument is Box<string>. They can be combined with constraints: <T extends object = {}>. That's how, for example, useState<S = undefined> and EventEmitter<T = any> work.

How do you make a generic parameter optional with a default value?

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