Case

> myText.toUpperCase()

Slicing

> myText.substr(first, length)

> myText.substring(start, end)

Replace character

> 'AAA'.replace(/A/g, 'C')
'CCC'

Or the more verbose.

> 'AAA'.replace(new RegExp("A", "g"), 'C')
'CCC'

You might use RegExp to escape characters for you.

> new RegExp("A", "g")
/A/g
> new RegExp("/A")
/\/A/

Warning

These will only replace the first occurence.

Using .replace without regex (like you would do in Python):

> 'AAA'.replace('A', 'C')
'CAA'

Using regex without g:

> 'AAA'.replace(/A/, 'C')
'CAA'

Remove empty lines

> myText.replace(/^\n/gm, '');

The m modified is recommended for multi-line search

https://www.w3schools.com/jsref/jsref_regexp_m.asp

Remove empty line from start and end

source

> myText.replace(/^\s+|\s+$/g, '');

Note this removes all whitespace and not just \n.

Template literals

See Template literals on Mozilla docs.