JavaScript - TypedArray findLastIndex() Method

Quiz

The JavaScript TypedArray findLastIndex() method iterates the typed array in reverse order, which means that the last element of the typed array will be considered as first element and vice versa. The method returns the index of the first element (from last) that satisfies the provided testing function. If none of the elements of the typed array satisfy the testing function, the method will return '−1'.

Here are some additional points you should know about the 'findLastIndex()' method −

The findLastIndex() method operates on a TypedArray (such as Uint8Array, Int16Array, etc.).
It accepts a testing function as a parameter.
The testing function is executed for each element in the TypedArray.
If an element satisfies the condition specified by the testing function (it returns a true value).
If no element satisfies the testing function −1 is returned.

Syntax

Following is the syntax of JavaScript TypedArray findLastIndex() method −

findLastIndex(callbackFn, thisArg)

Parameters

This method accepts two parameters named 'callbackFn' and 'thisArg', which are described below −

callbackFn − This parameter is a testing function that will be executed for each element in the TypedArray.

This function takes three parameters named 'element', 'index', and 'array'. Here's the description for each parameter −

element − Represents the current element being processed in the TypedArray.
index − Indicates the index (position) of the current element within the TypedArray.
array − Refers to the entire TypedArray.

thisArg (optional) − This is an optional parameter that allows you to specify the value of this within the callbackFn.

Return value

This method returns the index of the last element that satisfies the testing function. If no element satisfies the testing function, '−1' is returned.

Examples

Example 1

If the element satisfies the testing function, it will return the index of the last element in the typed array.

In the following example, we are using the JavaScript TypedArray findLastIndex() method to retrieve the index of the last element that satisfied the providing testing function. We create a testing function named isEven() checks for even numbers, and passes it as an argument to this method.

<html>
<head>
   <title>JavaScript TypedArray findLastIndex() Method</title>
</head>
<body>
   <script>
      function isEven(element, index, array){
         return element %2 == 0;
      }
      const T_array = new Uint8Array([10, 20, 30, 40, 50, 60, 70, 80, 81]);
      document.write("Typed array: ", T_array);
      
      //calling function
      document.write("<br>The index of the last even number is: ", T_array.findLastIndex(isEven));
   </script>
</body>
</html>

Output

The above program returns the index of the last even number as 7.

Typed array: 10,20,30,40,50,60,70,80,81
The index of the last even number is: 7

Example 2

Find the index of the last leap year in TypedArray.

In the given example below, we create a function named isLeapYear() that checks for leap year in this typed array [2002, 2004, 2005, 2006, 2007, 2014], and we pass this function as an argument to findLastIndex() method to retrieve the last leap year that satisfies the testing function.

<html>
<head>
   <title>JavaScript TypedArray findLastIndex() Method</title>
</head>
<body>
   <script>
      function isLeapYear(element, index, arrar){
         return ((element % 4 == 0) && (element % 400 == 0 || element % 100 !=0));
      }
      const T_array = new Int16Array([2002, 2004, 2005, 2006, 2007, 2012]);
      document.write("Typed array: ", T_array);
      
      //calling function
      document.write("<br>The index of the last leap year in typed array is: ", T_array.findLastIndex(isLeapYear));
   </script>
</body>
</html>

Output

Once the above program is executed, it will return the index of the last leap year in typed array as −

Typed array: 2002,2004,2005,2006,2007,2012
The index of the last leap year in typed array is: 5

Example 3

If no element satisfies the testing function, the findLastIndex() method will return '-1'.

The following is another example of the JavaScript TypedArray findLastIndex() method. We create a function named isNegative() that checks for the negative numbers, and passes it as an argument to this method to retrieve the index of the last element that satisfies the testing function.

<html>
<head>
   <title>JavaScript TypedArray findLastIndex() Method</title>
</head>
<body>
   <script>
      function isNegative(element, index, array){
         return element < 0;
      }
      const T_array = new Uint8Array([1, 2, 3, 4, 5, 6, 7, 8]);
      document.write("Typed array: ", T_array);
      
      //calling function
      document.write("<br>The index of last negative element: ", T_array.findLastIndex(isNegative));
   </script>
</body>
</html>

Output

After executing the above program, it will return '-1'.

Typed array: 1,2,3,4,5,6,7,8
The index of last negative element: -1

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