The MySQL2 driver return DECIMAL type as a string and formatCurrency only is for numbers. This is the only difference with the MySQL driver.
API and Configuration | Quickstart (sidorares.github.io)
There is no equivalent for DECIMAL in JavaScript, depending on the database driver the value is converted into a Number or a String. The Number type in JavaScript is a float and doesn’t have the same precision as the DECIMAL type of the database have and can cause data loss, so many database drivers return the value as a string instead.