notes.md

If

This challenge involves creating a TypeScript utility type If<C, T, F> that works in a similar manner to a ternary operator in JavaScript, but at the type level.

The ternary operator (condition ? valueIfTrue : valueIfFalse) takes three arguments: a condition, a value to return if the condition is true, and a value to return if the condition is false. The result is the value chosen based on the condition.

In TypeScript, we can achieve a similar effect using conditional types. A conditional type takes a condition (which is itself a type), and two types to choose between based on the condition. The syntax for a conditional type is type Conditional = Condition extends Base ? IfTrue : IfFalse.

In this case, C will be the condition (which is expected to be either true or false), T will be the type chosen if C is true, and F will be the type chosen if C is false.

The solution for this challenge will be:

type If<C extends boolean, T, F> = C extends true ? T : F

Here, C extends boolean ensures that C can only be true or false. Then, if C is true, the type will be T, otherwise, it will be F.

The extends keyword in the context of generic constraints means that the type parameter C can only be true or false, which is why If<null, 'a', 'b'> would lead to a TypeScript error.

In the type tests, we can see that If<true, 'a', 'b'> will resolve to 'a', and If<false, 'a', 2> will resolve to 2, as expected.