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T extends U ? X : Y

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type TypeName<T> =
T extends string ? "string" :
T extends number ? "number" :
T extends boolean ? "boolean" :
T extends undefined ? "undefined" :
T extends Function ? "function" :
"object";
type T1 = TypeName<string>;  // "string"
type T2 = TypeName<"hello">;  // "string"
type T3 = TypeName<123>;  // "number"
type T4 = TypeName<true>;  // "boolean"
type T5 = TypeName<undefined>;  // "undefined"
type T6 = TypeName<() => void>;  // "function"
type T7 = TypeName<object>;  // "object"
type T8 = TypeName<number[]>;  // "object"

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