N.J. Stat. § 2A:50-40
The holder of a mortgage, under the terms of which the mortgagor has agreed to pay the mortgagee a certain specified sum or sums of money at certain future time or times as therein specified, or as specified in the evidence of debt to secure which the mortgage was given, and also interest on the principal thereof during the term of the mortgage, at certain specified times at a rate not in excess of the legal annual rate, may, whenever the owner of the mortgaged premises defaults in making any such interest payment at the time when it has become due, or within such period of grace as is designated in the mortgage or the evidence of debt secured thereby, elect to foreclose the mortgage only to the extent that there is due and unpaid thereunder, upon the evidence of debt secured thereby, any interest upon the principal sum secured thereby.
N.J.S. § 2A:50-40