N.J. Stat. § 2A:82-1
In all cases where the genuineness of any signature or writing is in dispute, comparison of the disputed signature or writing with any writing proved to the satisfaction of the court to be genuine shall be permitted to be made by the witnesses; and such writings and the testimony of witnesses respecting the same may be submitted to the court or jury as evidence of the genuineness or otherwise of the signature or writing in dispute; provided nevertheless that where the handwriting of any person is sought to be disproved by comparison with other writings made by him, not admissible in evidence in the cause for any other purpose, such writings before they can be compared with the signature or writing in dispute, must, if sought to be used before the court or jury by the party in whose handwriting they are, be proved to have been written before any dispute arose as to the genuineness of the signature or writing in controversy.
N.J.S. § 2A:82-1