Node js is an open-source virtual machine that uses javascript as its scripting language. Despite being single-threaded, it is one of the most popular web technologies. The reason why node js is popular despite being single-threaded is the asynchronous nature that makes it possible to handle concurrency and perform multiple I/O operations at the same time. Node js uses an event loop to maintain concurrency and perform non-blocking I/O operations.
As soon as Node js starts, it initializes an event loop. The event loop works on a queue (which is called an event queue) and performs tasks in FIFO(First In First Out) order. It executes a task only when there is no ongoing task in the call stack. The call stack works in LIFO(Last In First Out) order. The event loop continuously checks the call stack to check if there is any task that needs to be run. Now whenever the event loop finds any function, it adds it to the stack and runs in order.
Let’s see an example to get a proper understanding of how to call stack and event loop works and handles concurrency,
Example : In this example, we will see the working of the call stack.
<script>
function add(a,b){
return a+b;
}
function print(n){
console.log(`Two times the number ${n} is `+add(n,n));
}
print(5);
</script>​
Output :
Two times the number 5 is 10
