10. How Does JS works #

Created Saturday 18 July 2020

• We don’t need to know how JS is implemented, but we do need to know how it works.

Sample Interview questions:

• How does JS actually work?
• Explain asynchronous vs synchronous.
• JS is single threaded that can be non-blocking, explain.

There are two basic mechanisms in a computer:

1. Memory Heap - Memory is limited, so we need a garbage collector - To avoid memory leaks(unused memory) - This is why global variables are discouraged.
2. Call stack - This is used to control the program flow, including recursion. Including function removal after execution.

• Stack overflow - There’s no space to include new activation records.

• What does single threaded mean? - JS has only one call stack. It is simple to implement as compared to multi-threaded. No deadlocks can occur.
• Synchronous - Code is run line by line.
• Synchronous can be problematic if a function takes too much time, e.g API calls, content load etc. This is also referred to as blocking.

Asynchronous helps us tackle potentially blocking LOC. How to program asynchronously?

1. Using a timeout for the blocking sections of the code. >

JavaScript Run-Time Environment

• In addition to the JS engine, we have Web APIs(like DOM, AJAX etc), a callback queue and an event loop. All this is referred to as the environment.

• timeout is provided by the browser.

How did the code run:

1. console.log(‘1’) got executed by the call stack.
2. The next line was a timeout, so Timeout API waited for 2 second, we were then ready to run the code.
3. The timeout code was sent to the Callback-queue(which is a queue of processes ready to run)
4. The event loop actually keeps on checking if the call stack is empty, and if it pops a process from the callback queue to the call stack.
5. During these 2 second, the call stack ran the code below line by line.

• Event listeners are persistent functions in the callback queue.

Practial examples of asynchronous:

1. Synchronous - We call a teacher and wait for the teacher to pick up and answer, but do noting while their phone is ringing. Asynchronous - We text the teacher and keep doing our work. The teacher responds when they are free.
2. Reading files, network requests.