The event loop is a fundamental concept in JavaScript that allows it to handle asynchronous operations. JavaScript is a single-threaded language, meaning that it can only execute one task at a time. However, the event loop allows JavaScript to give the illusion of multi-threading by using a few smart data structures.
Here are some key points about the event loop in JavaScript:
- The event loop is responsible for executing the code, collecting and processing events, and executing queued sub-tasks.
- At some point during the event loop, the runtime starts handling the messages on the queue, starting with the oldest one. To do so, the message is removed from the queue and its corresponding function is called with the message as an input parameter.
- The call stack is responsible for keeping track of all the operations in line to be executed. Whenever a function is finished, it is popped from the stack. The event queue is responsible for sending new functions to the stack for processing. Whenever an async function is called, it is sent to a browser API. These are APIs built into the browser. Based on the command received from the call stack, the API starts its own single-threaded operation.
- The event loop constantly checks whether or not the call stack is empty. If it is not empty, the event loop waits until it is empty and places the next function from the callback queue to the call stack. If the callback queue is empty, nothing will happen.
In summary, the event loop is a mechanism that allows JavaScript to handle asynchronous operations and give the illusion of multi-threading. It is responsible for executing the code, collecting and processing events, and executing queued sub-tasks. The call stack and event queue work together to ensure that functions are executed in the correct order.
