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Example of using Typescript keyof operator
interface RecursiveArray<T> extends Array<T | RecursiveArray<T>> {}
function getLast<O extends object, K extends keyof O>(
mapOrList: O | RecursiveArray<O>,
key: K
): O[K] | null {
if (!mapOrList) return null;
if (Array.isArray(mapOrList)) {
return getLast(mapOrList.reverse().find(m => !!getLast(m, key))!, key);
}
return mapOrList[key];
}
getLast({color: "green"}, "color" ) // "green"
getLast([{color: "red"}], "color"); // Argument of type '"color"' is not assignable to parameter of type 'never'
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