why is there a backslash before a comma in a string js code example

Example 1: how to fetch all chars of a string before a space in python

>>> s1.split(':')
['Username', ' How are you today?']
>>> s1.split(':')[0]
'Username'

Example 2: javascript how to deal with %20 in string

decodeURI(str)

Example 3: how to print any string in double quotes in php

$web = "...if (url.contains(\".mp4\"))..."