JavaScript Array findIndex() Method

Last Updated : 23 Jun, 2020

Below is the example of the Array findIndex() method to find the index of positive number from an array.

Example:

<script> 
    // input array contain some elements. 
    var array = [-10, -0.20, 0.30, -40, -50]; 
  
    // function (return element > 0). 
    var found = array.findIndex(function (element) { 
        return element > 0; 
    }); 
  
    // Printing desired values. 
    document.write(found); 
</script>                     

Output:
```
2
```

The arr.findIndex() method used to return the index of the first element in a given array that satisfies the provided testing function. Otherwise, -1 is returned.

It does not execute the method once it finds an element satisfying the testing method.
It does not change the original array.

Syntax:

array.findIndex(function(currentValue, index, arr), thisValue)

Parameters: This method accepts five parameters as mentioned above and described below:

function: It is the function of the array that works on each element.
currentValue: This parameter holds the current element.
index: It is an optional parameter that holds the index of the current element.
arr: It is an optional parameter that holds the array object the current element belongs to.
thisValue: This parameter is optional, if a value to be passed to the function to be used as its “this” value else the value “undefined” will be passed as its “this” value.

Return value:It returns the array element index if any of the elements in the array pass the test, otherwise it returns -1.

Below examples illustrate the Array findIndex() function in JavaScript:

Example 1: In this example the method findIndex() finds all the indices that contain odd numbers. Since no odd numbers are present therefore it returns -1.
```
function isOdd(element, index, array) {
  return (element%2 == 1);
}

print([4, 6, 8, 12].findIndex(isOdd)); 
```
Output:
```
-1
```
Example 2: In this example the method findIndex() finds all the indices that contain odd numbers. Since 7 is odd number therefore it returns its index
.
```
function isOdd(element, index, array) {
  return (element%2 == 1);
}

print([4, 6, 7, 12].findIndex(isOdd)); 
```
Output:
```
2
```

Codes for the above function are defined as follows:

Program 1:

<script> 
    // Input array contain elements 
    var array = [10, 20, 30, 110, 60]; 
  
    // Testing method (element > 25). 
    function finding_index(element) { 
        return element > 25; 
    } 
  
    // Printing the index of element which is satisfies 
    document.write(array.findIndex(finding_index)); 
</script> 

Output:

Program 2:

<script> 
    // Number is prime or not prime. 
    function isPrime(n) { 
        if (n === 1) { 
            return false; 
        } else if (n === 2) { 
            return true; 
        } else { 
            for (var x = 2; x < n; x++) { 
                if (n % x === 0) { 
                    return false; 
                } 
            } 
            return true; 
        } 
    } 
  
    // Printing -1 because prime number is not found. 
    document.write([4, 6, 8, 12].findIndex(isPrime)); 
    document.write("<br />") 
    // Printing 2 the index of prime number (7) found. 
    document.write([4, 6, 7, 12].findIndex(isPrime)); 
</script> 