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Discussion on: How to make Generic Functions that change return value type according to the parameter values type in TypeScript?

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yossarian • • Edited

Small improvement:

type Json = number | boolean | string | { [prop: string]: Json } | Array<Json>;

const getFirstElement = <Elem extends Json, Tuple extends Elem[]>(
  arr: [...Tuple]
): Tuple[0] => arr[0];

const value1 = getFirstElement([1, 2, 3, 4]); // 1

const value2 = getFirstElement(["hello", "hai", "hey"]); // "hello"
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As you might have noticed, TS is able to infer literal type with help of Variadic tuple types.
You can find more examples in my blog here and here