This is a summary of the official document
Conditional types are very self-explanatory. Depending on its condition, the type can be either one or the other.
T extends U ? X: Y
A conditional type T is either X, or, Y, or deferred depending on one or more type variables. The type of T depends on whether the type system has enough information to conclude that T is always assignable to U.
Look at the example below;
declare function f<T extends boolean>(x: T): T extends true ? string : number;
let x = f(Math.random() < 0.5);If I were to examine the code above, type T could be either string, number, or deferred. And it depends on whether the type system has enough information to conclude that T is always assignable to true in this case.
if Math.random() is a number less than 0.5, the type T is number.