如何获取数组数组的索引？

var arr = [1, 2, 3, 4, 5];
var target = 3;
var index = -1;

for (var i = 0; i < arr.length; i++) {
  if (arr[i] === target) {
    index = i;
    break;
  }
}

console.log("目标值的索引为：" + index);

var arr = [1, 2, 3, 4, 5];
var target = 3;
var index = arr.indexOf(target);

console.log("目标值的索引为：" + index);

var arr = [1, 2, 3, 4, 5];
var target = 3;
var index = arr.findIndex(function(element) {
  return element === target;
});

console.log("目标值的索引为：" + index);