Conditional Types

A ternary expression in JavaScript chooses between two values. A conditional type does the same job for types, and the two have nearly the same syntax.

const value: unknown = "hello";

// A ternary chooses a value at runtime
const label = typeof value === "string" ? "Yes" : "No";

// A conditional type chooses a type at compile time
type IsString<T> = T extends string ? "Yes" : "No";

type Result1 = IsString<string>;  // "Yes"
type Result2 = IsString<number>;  // "No"

The ternary runs when the program runs