CODEWITHSUNDEEP
typescript
CSSer
CSSer

Reputation: 2587

"extends boolean" and give default?

type Test<T extends boolean = false> = {
  a?: T;
};
const t: Test = {
  a: true,
};

The code above complains about Type 'true' is not assignable to type 'false'.ts(2322)

Is there any way to achieve what I want?

=========== update ===========

Thanks for the comments and answers!

I thought when declaring a generic type, that T is determined by the property that was given the T.

So if I assign a: true, that T becomes true; if I assign a: false or undefined, the T uses the default false.

Just like what function parameters and the defaults do.

So I have to explicitly tell ts Test<true>?

What about this use case:

type RecursiveType<OK extends boolean = false> = { // 1
  isOK: OK;
  property: OK extends true ? ("a"|"b") : 0;
  children?: Array<RecursiveType>; // 2
}

const recursive:RecursiveType = {
  isOK: true;
  property: 'a',
  children: [
    {
      isOK: false,
      property: 0
    }
  ]
}

How to declare the type properly so my editor (vscode) give correct hints depends on isOK?

Upvotes: 0

Views: 738

Answers (1)

Matthieu Riegler
Matthieu Riegler

Reputation: 55152

If you don't pass any type to the generic it will use the default value, here false.

You can either do

const t: Test = { // no type === will only accept false
  a: false,
};

or

const t: Test<true> = { // will only accept the type passed to the generic
  a: true,
};

Upvotes: 2

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