why request.readyState === 4 ? why 4 ?

Hey Ekramy M.Ali,

4 is one of the value for the readyState property. The full list of readyState values is:

0 - The request is initialized

1 - The request has been set up

2 - The request has been sent

3 - The request is in process

4 - The request is completed

if( xhr.readyState === 4 ) {
// Run this code
}

The above conditional statement checks, if the value is 4 which means if the request is complete, run the block of code. We can use other values (numbers) too but it will come up as incomplete or get an error whilst processing therefore we rarely use other numbers.

To learn more about this property and how it works, check out this stackoverflow: https://stackoverflow.com/questions/30522565/what-is-meaning-of-xhr-readystate-4

Check out this MDN, great resource: https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/readyState

Hope this helps!