How to replace all characters in a string using JavaScript for this specific case: replace . by _

The . character in a regex will match everything. You need to escape it, since you want a literal period character:

var s1 = s2.replace(/\./gi, '_');

you need to escape the dot, since it's a special character in regex

s2.replace(/\./g, '_');

Note that dot doesn't require escaping in character classes, therefore if you wanted to replace dots and spaces with underscores in one go, you could do:

s2.replace(/[. ]/g, '_');

Using i flag is irrelevant here, as well as in your first regex.

You can also use strings instead of regular expressions.

var s1 = s2.replace ('.', '_', 'gi')