Singlethread js concept.txt

JavaScript is single-threaded, which means it has only one call stack and can only do one thing at a time. This is an important concept because it affects how JavaScript handles tasks, especially asynchronous operations. Here’s a detailed explanation of how single-threaded JavaScript works:

The Single Threaded Nature of JavaScript

1. Call Stack
The call stack is a data structure that keeps track of the function calls in the program. When a function is called, it's added to the top of the stack. When the function returns a value, it's removed from the stack.
If the stack is busy (e.g., executing a function), no other function can be executed until the stack is empty again.


2. Blocking and Non-Blocking Code
Blocking code: Any code that causes the call stack to wait. For example, synchronous functions like alert(), loops that run indefinitely, etc.
Non-blocking code: Code that doesn't block the execution of subsequent code, often achieved using asynchronous operations like setTimeout(), fetch(), Promises, and async/await.


3. Event Loop
The event loop is what allows JavaScript to perform non-blocking operations despite being single-threaded.
It continuously checks the call stack and the task queue. If the call stack is empty, it pushes the first task from the task queue to the call stack.


4. Task Queue (Callback Queue)
When asynchronous operations complete, their callbacks are added to the task queue.
The event loop takes these callbacks from the task queue and pushes them to the call stack when the call stack is empty.


5. Microtask Queue
Microtasks, such as those created by Promise callbacks (then, catch, finally), are placed in a separate queue called the microtask queue.
The event loop checks the microtask queue before moving to the task queue. This means microtasks have a higher priority and are executed before tasks from the task queue.