數組找索引

const indexIwant = (arr, val) => arr.reduce((acc, el, i) => (el === val ? [...acc, i] : acc), []);
indexIwant([1,2,3,1,2,3], 2) // 輸出[1, 4]
indexIwant([1,2,3], 4) // 輸出[]

indexIwant(arr, 想要找索引的元素)