如何斷言字符串可以用作數組索引，而不將其轉換/斷言為數字？

我有一個字符串值，我打算將它用作數組的索引。我確信這個值，并希望斷言它是一個有效的“索引值”，在運行時將其轉換為數字或斷言它是數字（b/c它不是數字）。

所以我有：

const inputEl = window.document.getElementById('foo') as HTMLInputElement;
//it's indeed a string, I'm sure of it and have no squirm because of it
const v = inputEl.value as string;
const a = ['foo',];
const result = a[v];

console.log(result);

當前tsc次投擲

Element implicitly has an 'any' type because index expression is not of type 'number' (tsserver 7015)

我寧愿不做這樣的事情：

//just because it's not a number
const v = inputEl.value as unknown as number;

有更好的方法嗎？非常感謝。