My Calculator.js

JavaScript implementation of a basic calculator. Looking for constructive criticism of my code. Thanks in advance!

    window.onload = function() {

    // number button variable declarations

    var zeroButton = document.getElementById("zero");
    var oneButton = document.getElementById("one");
    var twoButton = document.getElementById("two");
    var threeButton = document.getElementById("three");
    var fourButton = document.getElementById("four");
    var fiveButton = document.getElementById("five");
    var sixButton = document.getElementById("six");
    var sevenButton = document.getElementById("seven");
    var eightButton = document.getElementById("eight");
    var nineButton = document.getElementById("nine");

    // operator button variable declarations

    var plusButton = document.getElementById("plus");
    var minusButton = document.getElementById("minus");
    var timesButton = document.getElementById("times");
    var dividedByButton = document.getElementById("divided_by");

    // action button variable declarations

    var clearButton = document.getElementById("clear");
    var enterButton = document.getElementById("enter");

    // display variable declarations

    var operatorDisplay = document.getElementById("operator_display");
    var resultDisplay = document.getElementById("result_display_value");

    // variable holds hidden number

    var hiddenNum = "";

    // number button function

    var numberFunction = function(numberButton) {
        numberButton.onclick = function(event) {
            resultDisplay.innerHTML += numberButton.innerHTML;
        }
    };

    // number function calls

    numberFunction(zeroButton);
    numberFunction(oneButton);
    numberFunction(twoButton);
    numberFunction(threeButton);
    numberFunction(fourButton);
    numberFunction(fiveButton);
    numberFunction(sixButton);
    numberFunction(sevenButton);
    numberFunction(eightButton);
    numberFunction(nineButton);

    // operations function

    var operations = function() {
        if (operatorDisplay.innerHTML === "+") {
            resultDisplay.innerHTML = parseInt(hiddenNum) + parseInt(resultDisplay.innerHTML);
        } else if (operatorDisplay.innerHTML === "-") {
            resultDisplay.innerHTML = parseInt(hiddenNum) - parseInt(resultDisplay.innerHTML);
        } else if (operatorDisplay.innerHTML === "*") {
            resultDisplay.innerHTML = parseInt(hiddenNum) * parseInt(resultDisplay.innerHTML);
        } else if (operatorDisplay.innerHTML === "/") {
            resultDisplay.innerHTML = parseInt(hiddenNum) / parseInt(resultDisplay.innerHTML);
        }
    };

    // operator button function

    var operatorFunction = function(operatorButton) {
        operatorButton.onclick = function(event) {
            if (resultDisplay.innerHTML !== "" && hiddenNum !== undefined) {
                operations();
            }
            hiddenNum = resultDisplay.innerHTML;
            resultDisplay.innerHTML = "";
            operatorDisplay.innerHTML = operatorButton.innerHTML;
        }
    };

    // operator function calls

    operatorFunction(plusButton);
    operatorFunction(minusButton);
    operatorFunction(timesButton);
    operatorFunction(dividedByButton);

    // clear function

    var clearFunction = function(button, display) {
        display.innerHTML = "";
    };

    // clear button

    clearButton.onclick = function(event) {
        clearFunction(clearButton, operatorDisplay);
        clearFunction(clearButton, resultDisplay);
        hiddenNum = "";
    }

    // enter button

    enterButton.onclick = function(event) {
        if (resultDisplay.innerHTML === "" && hiddenNum === "") {
            resultDisplay.innerHTML = "";
        } else if (resultDisplay.innerHTML === "") {
            if (operatorDisplay.innerHTML === "+") {
                resultDisplay.innerHTML = parseInt(hiddenNum) + parseInt(hiddenNum);
            } else if (operatorDisplay.innerHTML === "-") {
                resultDisplay.innerHTML = parseInt(hiddenNum) - parseInt(hiddenNum);
            } else if (operatorDisplay.innerHTML === "*") {
                resultDisplay.innerHTML = parseInt(hiddenNum) * parseInt(hiddenNum);
            } else if (operatorDisplay.innerHTML === "/") {
                resultDisplay.innerHTML = parseInt(hiddenNum) / parseInt(hiddenNum);
            }
        } else {
            operations();
        }
        clearFunction(enterButton, operatorDisplay);
    }

}