JSFiddle - React, Tailwind, and code Playground

JavaScript

var $scope = {
    operator: '-',
    answer: null,
    leftOperand: null,
    rightOperand: 3,
};

$scope.setOperand = function (operandEntered) {
    [['leftOperand','rightOperand'].reduce(function(t, k) {
        return $scope[k] === null ? k : t;
    })].map(function(prop){$scope[prop] = +$scope[prop] + operandEntered});
};

$scope.setOperand(6);

console.log($scope);