**arr.indexOf()** function is used to find the index of the first occurrence of the search element provided as the argument to the function. The syntax of the function is as follows:

arr.indexOf(searchElement[,index])

**Arguments**

The first argument to this function is the **searchElement** which is the value to be searched in the array. Second argument to this function is the optional **index** argument which defines the starting index in the array from where the element is to be searched. If this argument is not provided then index **0** is taken as the starting index to begin the search as it is the default value.

**Return value**

This function returns the index of the first occurrence of the **searchElement**. If the element cannot be found in the array, then this function returns **-1**

*Examples for the above function are provided below:*

**Example 1:** In this example the function **indexOf()** finds the index of first occurrence of **2** in the array. Since **2** is present at position **0**, therefore it returns it as the answer.

var array = [2, 9, 9]; print(array.indexOf(2));

Output:

0

**Example 2:** In this example the function **indexOf()** checks for **45** from index **2** in the array. Since it is found at position **3**, therefore it returns it as the answer.

var array = [2, 98, 12, 45]; // Here second argument is starting index // from where we need to search. print(array.indexOf(45, 2));

Output:

3

**Example 3:** In this example the function **indexOf()** checks for **98** from index **2** in the array. Since it is not found, therefore it returns **-1** as the answer.

var array = [2, 98, 12, 45]; print(array.indexOf(98,2));

Output:

-1

**Example 4:** In this example the function **indexOf()** checks for **100** in the array. Since it is not found, therefore it returns **-1** as the answer.

var array = [2, 98, 12, 45]; print(array.indexOf(100));

Output:

-1

**Example 5:** In this example the function **indexOf()** checks for **98** in the array. Since it is found at position **1**, therefore it returns **1** as the answer.

var array = [2, 98, 12, 98]; print(array.indexOf(98));

Output:

1

*Codes for the above function are provided below:*

**Program 1:**

`// JavaScript to illustrate indexOf() function ` `<script> ` `function` `func() { ` ` ` `var` `array = [2, 9, 9]; ` ` ` `document.write(array.indexOf(2)); ` `} ` `func(); ` `</script> ` |

Output:

0

**Program 2:**

`// JavaScript to illustrate indexOf() function ` `// with two parameters. ` `<script> ` `function` `func() { ` ` ` `var` `array = [2, 98, 12, 45]; ` ` ` ` ` `// Here second argument is starting index ` ` ` `// from where we need to search. ` ` ` `document.write(array.indexOf(45, 2)); ` `} ` `func(); ` `</script> ` |

Output:

3

**Program 3:**

`//JavaScript to illustrate indexOf() function ` `<script> ` `function` `func() { ` ` ` `var` `array = [2, 98, 12, 45]; ` ` ` `document.write(array.indexOf(98,2)); ` `} ` `func(); ` `</script> ` |

Output:

-1

**Program 4:**

`//JavaScript to illustrate indexOf() function ` `<script> ` `function` `func() { ` ` ` `var` `array = [2, 98, 12, 45]; ` ` ` `document.write(array.indexOf(100)); ` `} ` `func(); ` `</script> ` |

Output:

-1

**Program 5:**

`//JavaScript to illustrate indexOf() function ` `<script> ` `function` `func() { ` ` ` `var` `array = [2, 98, 12, 98]; ` ` ` `document.write(array.indexOf(98)); ` `} ` `func(); ` `</script> ` |

Output:

1

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