Atomics.sub( ) In JavaScript

What is Atomics?

The Atomics is an object in JavaScript which provides the ability to perform atomic operations as static methods.
Just like the Math object in JavaScript all the properties and methods of Atomics are also static.
Atomics are used with SharedArrayBuffer(generic fixed-length binary data buffer) objects.
Atomics are not constructors like other global objects.
Atomics cannot be used with a new operator or can be invoked as a function.

Atomics Operations in JavaScript
Multiple threads can read and write the same data in memory when there is shared memory. To ensure that predicted values are written and read accurately, another operation cannot start until and unless the current one finishes. Atomic operations also cannot be interrupted.

Atomics.sub() Method
Among the Atomic Operations, there is a method Atomics.sub() that is used to subtract a given value at a given position in the array and returns the old value at that position. No other write operation can happen until the modified value is written back.

Syntax:

Atomics.sub(typedArray, index, value)

Parameters Used:

typedarray: It is the shared integer typed array you want to modify.
index: It is the position in the typedArray from where you want to subtract a value from.
value : It is the number of you want to subtract.

Return Value:

Atomics.sub() returns the old value at the given position (typedArray[index]).

Examples of the above function are provided below.

Examples:

Input : arr[0] = 9;
        Atomics.sub(arr, 0, 3);
Output : 9

Input : arr[0] = 3; 
        Atomics.sub(arr, 0, 2);
Output : 3

Codes for the above function are provided below.

Code 1:

<script> 
// creating a SharedArrayBuffer 
var buf = new SharedArrayBuffer(25); 
var arr = new Uint8Array(buf); 
// Initialising element at zeroth position of array with 9 
arr[0] = 9; 
  
// Displaying the return value of the Atomics.sub() method  
console.log(Atomics.sub(arr, 0, 3)); 
  
// Displaying the updated SharedArrayBuffer  
console.log(Atomics.load(arr, 0)); 
</script> 

OUTPUT :

9
6

Code 2:

<script> 
// creating a SharedArrayBuffer  
var buf = new SharedArrayBuffer(25); 
var arr = new Uint8Array(buf); 
  
// Initialising element at zeroth position of array with 3 
arr[0] = 3; 
  
// Displaying the return value of the Atomics.sub() method  
console.log(Atomics.sub(arr, 0, 2)); 
  
// Displaying the updated SharedArrayBuffer  
console.log(Atomics.load(arr, 0)); 
</script> 

OUTPUT :

3
1

Application:
Whenever we want to update(subtract) a value at a specified position in a given array and want the older value which was at that position to be returned, we use the Atomics.sub() operation in JavaScript.

Let’s see a JavaScript Program :

<script> 
// creating a SharedArrayBuffer  
var mybuffer = new SharedArrayBuffer(25); 
var myarray = new Uint8Array(mybuffer); 
  
// Initialising the element at zeroth position of array 
myarray[0] = 40; 
  
// Displaying the return value of the Atomics.sub() method 
console.log(Atomics.sub(myarray, 0, 20)); 
  
// Displaying the updated SharedArrayBuffer  
console.log(Atomics.load(myarray, 0)); 
</script> 

Output :

40
20

Exceptions :

If the typedArray is not one of the allowed integer types then the Atomics.sub() operation throws a TypeError.
If the typedArray is not a shared typed array then the Atomics.sub() operation throws a TypeError.
If the index used as an argument to the Atomics.sub() operation is out of the bound in the typedArray then the Atomics.sub() operation throws a RangeError.

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