How do you use the indexOf method in JavaScript?

The indexOf() method in JavaScript is used to find the index position of a specified element in an array. It returns the index value of the element if found, or -1 if not found.

Grammar: Syntax

array.indexOf(item, start)

Parameter:

1. element to be searched for
2. start: optional parameter that specifies the index position to start searching from, with a default value of 0.

示例：Example

var fruits = ["apple", "banana", "orange", "grape"];
var index = fruits.indexOf("orange");
console.log(index); // 输出：2

var index2 = fruits.indexOf("watermelon");
console.log(index2); // 输出：-1

In the example above, return the index value of “orange” if it exists in the array fruits; return -1 if “watermelon” is not in the array.