The event loop in JavaScript is a mechanism that allows the execution of code to be scheduled and managed in a non-blocking way. It is what enables JavaScript to handle multiple tasks at the same time, such as user input, network requests, and timers, without freezing the browser or the JavaScript engine.
The event loop works by constantly checking the message queue, which is a list of messages, or tasks, that need to be executed. Each message, or task, is associated with a callback function that will be executed when the message is processed.
The message queue is processed in a first-in, first-out (FIFO) order, and the JavaScript engine only processes one message at a time. As soon as the engine starts processing a message, it checks the message queue again to see if there are any new messages to process. If there are, the engine will start processing the next message, and so on, until the message queue is empty.
The event loop also handles the execution of web APIs, such as timers and network requests. These web APIs are non-blocking, which means that they do not block the execution of the JavaScript engine. Instead, they add a message to the message queue with a callback function that will be executed once the web API has completed its task.
The event loop is a continuous process, it starts when the JavaScript engine starts and continues running until the page is closed or the JavaScript engine is stopped.
In summary, the event loop in JavaScript is a mechanism that allows for the scheduling and management of code execution in a non-blocking way. It enables JavaScript to handle multiple tasks at the same time by constantly checking the message queue, processing messages in a FIFO order and handling execution of web APIs such as timers and network requests. It is a continuous process that starts when the JavaScript engine starts and continues running until the page is closed or the JavaScript engine is stopped.