function findIndex(list, item) {
  for (let i in list) {
    if (list[i] == item) {
      return i;
    }
  }

  // 没有找到元素返回-1
  return -1;
}

const list = ["a", "b", "c"];
let item = "b";

let index = findIndex(list, item);
console.log(index); // 1