What does this TypeScript snippet output?

Console output

Why

The well-known symbol `Symbol.toPrimitive` lets an object define its own conversion to a primitive. When the unary `+` operator coerces `o` to a number, it calls that method and gets `42`, bypassing the usual `valueOf`/`toString` ladder. TypeScript types `Symbol.toPrimitive` as a `unique symbol`, so it can be used as a computed key with full type awareness.