It needs to be able to both sign and verify. Only thing I've found just signs.
Curve25519 implements Diffie-Hellman key exchange; it doesn't sign or verify anything, it only generates a shared key via scalar multiplication. If you're looking for signatures, use Ed25519, which is an implementation of Schnorr signature algorithm on the same elliptic curve.
You can use this (https://github.com/rev22/curve255js) implementation