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Asynchronous JavaScript
JavaScript is single-threaded but it can still take advantage of asynchronous programming.
When we say JavaScript is single-threaded, we mean that we cannot write
multithread code without using JavaScript APIs. In fact,
despite for us it appears single-threaded, it takes advantage of the
libuv library, allowing the engine to be effectively multithread.
In order to do that, JavaScript uses
- The libuv APIs which allow multithreading
- The Event Loop Queue
Explaination
JavaScript provides a set of APIs, including setTimeout and fetch, which
are offloaded to the libuv library when called. The libuv library handles the
underlying operations, such as timer management and HTTP requests, respectively.
Both setTimeout and fetch have associated functions, known as callbacks, that
are executed once the operation has completed.
Since the engine cannot directly push these callback functions onto the call stack without disrupting the current flow of execution, it instead places them in a queue called the "Event Loop Queue". When the call stack is empty, the engine checks the queue and, if there are any pending callbacks, proceeds to execute them.
Example
setTimeout(() => console.log('I will go in the event loop queue first'), 1000);
console.log('I am in the stack');
What is happening here? setTimeout is a JavaScript API which will be taken care
of the libuv library which will be responsible for checking when 1000ms have passed.
Concurrently to the libuv library doing what it has to do, the engine executes
console.log('I am in the stack');.
When 1000ms have elapsed, the callback function associated with the timer (console.log)
gets enqueued in the Queue.
Since there is nothing to do in the stack, the callback pushes the callback onto the
stack and executes it.