What is the use of -1 in instructorAccts.push(_address) -1?

function setInstructor(address _address, uint _age, string _fName, string _lName) public {
        var instructor = instructors[_address];
        
        instructor.age = _age;
        instructor.fName = _fName;
        instructor.lName = _lName;
        
        instructorAccts.push(_address) -1;
    }