JavaScript 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 the 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 operations return the older value which was present at the specified position.

Syntax:

Atomics.exchange(typedArray, index, value)

Parameters:

• 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 exchange a value.
• 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

Examples of the above function are provided below.

Example 1:

javascript

// creating a SharedArrayBuffer
let buf = new SharedArrayBuffer(25);
let 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));

Output :

9
2

Example 2:

javascript

// creating a SharedArrayBuffer
let buf = new SharedArrayBuffer(25);
let 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));

Output :

3
1

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

Example:

javascript

// creating a SharedArrayBuffer
let mybuffer = new SharedArrayBuffer(25);
let 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));

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.

Supported Browser:

• Google Chrome
• Microsoft Edge
• Firefox
• Opera
• Safari

We have a complete list of Javascript Atomic methods, to check those please go through this JavaScript Atomics Complete Reference article.

We have a Cheat Sheet on Javascript where we covered all the important topics of Javascript to check those please go through Javascript Cheat Sheet-A Basic guide to JavaScript.