使用 JavaScript 的逻辑运算符压缩 if/else if 语句

我正在学习 AngularJS，试图制作一个简单的计算器，并且我正在尝试压缩它if/else if 陈述使用 Javascript 逻辑运算符 (&&, ||, !)

鉴于这个例子，我怎样才能减少它呢？ （如果你不明白$scope， 忽略它。它基本上是一个视图。因此，当有人点击 9 时，计算器将在答案屏幕上显示 9）

$scope.setOperand = function (operandEntered) {

    if ($scope.leftOperand === null) {
        $scope.leftOperand = operandEntered;
    } 
    else if ($scope.operator === null) {
        $scope.leftOperand = $scope.leftOperand + operandEntered;
    } 
    else if ($scope.rightOperand === null) {
        $scope.rightOperand = operandEntered;
    } 
    else if ($scope.answer === null) {
        $scope.rightOperand = $scope.rightOperand + operandEntered;
    }
 };

人们总是可以（尝试）变得聪明，但是当代码变得更长并且不更可重用时，抽象并不总是有回报。KISS https://en.wikipedia.org/wiki/KISS_principle.

但如果你想这么做的话我会选择

function addTo(property, x, operand) { // "x" for lack of a more meaningful name
    const shouldAdd = $scope[property] === null || $scope[x] === null;
    if (shouldAdd)
        $scope[property] += operand; // assuming "operand" is a number
// or   $scope[property] = ($scope[property] || "") + operand; // when it's a string
    return shouldAdd;
}
$scope.setOperand = function (operandEntered) {
    addTo("leftOperand", "operator", operandEntered) || addTo("rightOperand", "answer", operandEntered);
 };

如果您更关心简洁性而不是可读性，您甚至可以将助手缩短为

function addTo(property, x, operand) {
    return ($scope[property] === null || $scope[x] === null) && (($scope[property] += operand), true);
}