npm install elliptic-curve
Import the secp256k1 module:
> var secp256k1 = require('elliptic-curve').secp256k1
> var privateKey = '278a5de700e29faae8e40e366ec5012b5ec63d36ec77e8a2417154cc1d25383f'
> secp256k1.getPublicKey(privateKey)
'03fdd57adec3d438ea237fe46b33ee1e016eda6b585c3e27ea66686c2ea5358479'
> var message = "Hello, world!",
> secp256k1.signMessage(message, privateKey)
'3046022100997b6210d959e67ad9cee01589d01daf0fe77ce0f002d040d769171c33504860022100e35a03d2354074d7e49d0499568e331be39af901a543d1731ea1ff8f423f21ab'