Atomics.exchange( ) 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 shared memory is. 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.exchange() Method

  • Among the Atomic Operations, there is an inbuilt operation Atomics.exchange() that is used to exchange and store a new value at a specific position in an array.
  • Atomics.exchange( ) operation in JavaScript returns the old value at that position of the array which has been exchanged with a new value.
  • No other write operation can happen between the read of the old value and write of the new value.

Difference between Atomics.compareExchange and Atomics.exchange
As we read that the Atomics.compareExchange( ) operation in JavaScript exchanges the value in the array only if the passed parameter is equal to the old value at that position whereas the Atomics.exchange() operation in JavaScript exchanges and stores a given value at a given position in the array without comparing it with the older value residing at that position of the array.
Both the operations return the older value which was present at the specified position.

Syntax:



Atomics.exchange(typedArray, index, value)

Parameters Used:

  1. typedarray: It is the shared integer typed array you want to modify.
  2. index:It is the position in the typedArray from where you want to exchange a value.
  3. value:It is the number to exchange.

Return Value:
Atomics.exchange() returns the old value at the given position (typedArray[index]).

Examples of the above function are provided below.

Examples:

Input : arr[0] = 9;
        Atomics.exchange(uint8, 0, 2);
Output : 2
Input : arr[0] = 3; 
        Atomics.exchange(uint8, 0, 1);
Output : 1

Codes for the above function are provided below.

Code 1:

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<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 SharedArrayBuffer 
console.log(Atomics.load(arr, 0));
  
// Exchanging value in the SharedArrayBuffer using the Atomics.exchange() method 
Atomics.exchange(arr, 0, 2);
  
// Displaying the updated SharedArrayBuffer 
console.log(Atomics.load(arr, 0));
  
</script>

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OUTPUT :

9
2

Code 2:

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<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 SharedArrayBuffer
console.log(Atomics.load(arr, 0));
  
// Exchanging value in the SharedArrayBuffer using the Atomics.exchange() method 
Atomics.exchange(arr, 0, 1);
  
// Displaying the updated SharedArrayBuffer 
console.log(Atomics.load(arr, 0));
</script>

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OUTPUT :

3
1

Application:
Whenever we want to exchange a value at a specific position of an array and also want to return the older value which was at that position of the array, we use the Atomics.exchange() operation in JavaScript.

Let’s see a JavaScript Program :

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<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.exchange() method
console.log(Atomics.exchange(myarray, 0, 20));
  
// Displaying the updated SharedArrayBuffer
console.log(Atomics.load(myarray, 0));
</script>

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Output :

40
20

Exceptions :

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


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