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async.queue() Method in Node.js
The async module provides different functionalities to work with asynchronous JavaScript in a nodejs application. The async.queue() method returns a queue that is further used for concurrent processing of processes i.e. multiple processing of items at a time/instant.
Installing and Using async.queue()
Step 1 − Run the following command to initialize the node package manager.
npm init
Step 2 − Installing the async module using the following command.
npm install --save async
Step 3 − Importing the async module using the below statement in your programs .
const async = require('async')Syntax
async.queue('function', 'concurrency value')Parameters
The above parameters are described as below −
function – This parameter defines the function that will be executed over the element added to the queue.
concurrency value – This field defines the number of elements to be processed at a time.
The async.queue() method further has multiple methods and properties that will be used while processing async requests.
push(element, callback) - Similar to an ordinary queue, the push method is used for adding an element at the tail of a queue.
queue.push(item, callback);
length() - The length method is used for returning the number of elements present in a queue at a time.
queue.length()
started property - This property returns a boolean value providing information about the queue whether it has started processing its elements or not.
queue.started()
unshift(element, callback) - The unshift property also adds an element to the queue like a push() method. The only difference between the two is – It adds elements at the head whereas push adds it in the tail. This method is used for priority elements.
queue.unshift(item, callback)
drain() Method - This method issues a callback when the queue has executed all the tasks/elements. It only works when the function is described in an arrow function.
queue.drain(() => {
console.log(“All Tasks are completely executed...”);
}pause() Method This method holds the execution of remaining elements in the queue. The function will continue after resume() is called.
queue.pause()
resume() Method - This method is used for resuming the execution of elements which were put on hold using the pause() method.
queue.resume()
kill() Method - This method removes all the remaining elements from the queue and forces it into an idle state.
queue.kill()
idle() Method - This method returns a boolean state indicating if the queue is idle or processing something.
queue.idle
Example
Let's take a look at one example to understand the above concepts better −
// Including the async module
const async = require('async');
// Creating an array for all elements execution
const tasks = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
// Initializing the queue
const queue = async.queue((task, executed) => {
console.log("Currently Busy Processing Task " + task);
setTimeout(()=>{
// Number of tasks remaining and to be processed
const tasksRemaining = queue.length();
executed(null, {task, tasksRemaining});
}, 1000);
}, 1); // concurrency value = 1
// Queue is idle initially as no elements are there...
console.log(`Queue Started ? ${queue.started}`)
// Adding each task from the tasks list
tasks.forEach((task)=>{
// Adding the task 5 to the head for priority execution
if(task == 5){
queue.unshift(task, (error, {task, tasksRemaining})=>{
if(error){
console.log(`An error occurred while processing task ${task}`);
}else {
console.log(`Finished processing task ${task}. ${tasksRemaining} tasks remaining`);
}
})
// Adding all the tasks at tail to be executed except task 5
} else {
queue.push(task, (error, {task, tasksRemaining})=>{
if(error){
console.log(`An error occurred while processing task ${task}`);
}else {
console.log(`Finished processing task ${task}. ${tasksRemaining}
tasks remaining`);
}
})
}
});
// Executes the callback when the queue is done processing all the tasks
queue.drain(() => {
console.log('All items are succesfully processed !');
})
// Checking if the queue is started after adding tasks
console.log(`Queue Started ? ${queue.started}`)Output
C:\home
ode>> node asyncQueue.js Queue Started ? False Queue Started ? True Currently Busy Processing Task 5 Finished processing task 5. 9 tasks remaining Currently Busy Processing Task 1 Finished processing task 1. 8 tasks remaining Currently Busy Processing Task 2 Finished processing task 2. 7 tasks remaining Currently Busy Processing Task 3 Finished processing task 3. 6 tasks remaining Currently Busy Processing Task 4 Finished processing task 4. 5 tasks remaining Currently Busy Processing Task 6 Finished processing task 6. 4 tasks remaining Currently Busy Processing Task 7 Finished processing task 7. 3 tasks remaining Currently Busy Processing Task 8 Finished processing task 8. 2 tasks remaining Currently Busy Processing Task 9 Finished processing task 9. 1 tasks remaining Currently Busy Processing Task 10 Finished processing task 10. 0 tasks remaining All items are succesfully processed !
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