It’s because $$ inserts a literal “$“.
So, you need to use:
a = "aman/gupta";
a = a.replace("/", "$$$$"); // "aman$$gupta"
See the following special patterns:
| Pattern | Inserts |
|---|---|
$$ |
Inserts a “$“. |
$& |
Inserts the matched substring. |
$` |
Inserts the portion of the string that precedes the matched substring. |
$' |
Inserts the portion of the string that follows the matched substring. |
$n |
Where n is a non-negative integer less than 100, inserts the _n_th parenthesized submatch string, provided the first argument was a RegExp object. |
$<Name> |
Where Name is a capturing group name. If the group is not in the match, or not in the regular expression, or if a string was passed as the first argument to replace instead of a regular expression, this resolves to a literal (e.g., “$<Name>“). |