Javascript пишет, что DOM не работает

/**
 * core
 */
var display = null;
var input = null;

var latestNumber = null;
var result = null;
var displayNumber = null;
var operator = null;

function setOperator(operatorValue) {

    if (displayNumber === null && operator === null){
        displayNumber = readInput();
        writeOutput(displayNumber + " " + String(operatorValue));
        clearInput();

    } else if (displayNumber !== null && operator !== null){
        writeOutput( displayNumber + " " + String(operatorValue));
    }

    operator = String(operatorValue);


}

function addDigit(digit){
    let oldValue = readInput();

    if (Number.isNaN(oldValue)){
        oldValue = "";
    }

    let newInputValue = String(oldValue) + String(digit); //Parse numbers to string

    writeInput(newInputValue);
}

function calc (firstNumber, op, secondNumber){

    switch (op) {
        case "+":
            result = firstNumber + secondNumber;
            break;

        case "-":
            result = firstNumber - secondNumber;
            break;

        case "*":
            result = firstNumber * secondNumber;
            break;

        case "/":
            if (secondNumber === 0) {
                window.alert("Error: Division by zero");
            }
            else {
                result = firstNumber / secondNumber;
            }
            break;
    }

    operator = null;
    latestNumber = null;
    displayNumber = null;

    writeInput(result);
}

/**
 * UI
 */


window.addEventListener('load', function() {

    //Initialize number Buttons
    (function () {
        const numbers = document.getElementsByClassName("number");

        for (let i = 0; i < numbers.length; ++i) {
            numbers[i].addEventListener('click', () => addDigit(numbers[i].value));
        }
    })();

    //Initialize operation Buttons
    (function () {
        const operators = document.getElementsByClassName("operator");
        for (let i = 0; i < operators.length; ++i) {
            operators[i].addEventListener('click', () => setOperator (operators[i].value));
        }
    })();

    //Initialize calc button
    (function () {
        const equalButton = document.getElementById("key- = ");
        equalButton.addEventListener('click', ()=> calc (displayNumber, operator, readInput()));
    })();

    //Initialize clear button
    (function () {
        const commands = document.getElementsByClassName("command");
        for (let i = 0; i < commands.length; ++i) {
            commands[i].addEventListener('click', ()=> clearAll ());
        }
    })();

    //Initialize Display
    display = (function (){
        return document.getElementById("output");
    })();
    input = (function () {
        return document.getElementById("input");
    })();
    writeOutput("Welcome");

});

//Function to read from input
function readInput () {
    return parseFloat(input.value);
}

//Function to write to input
function writeInput (value) {
    input.value = value;
}

//Function to write to output
function writeOutput (string) {
    display.value = string;
}

//Function to clear input
function clearInput () {
    input.value = "";
}

//Function to clear output
function clearOutput () {
    display.value = "";
}

//Function to clear display
function clearAll (){
    result = null;
    lastNumber = null;
    displayNumber = null;
    operator = null;

    clearInput();
    clearOutput();
}

/* reset */
*  {
	-webkit-box-sizing: border-box;
	-moz-box-sizing: border-box;
	-ms-box-sizing: border-box;
	box-sizing: border-box;
	margin: 0;
	padding: 0;
}

html {
	font-size: 1vw;
}

html, body {
	width: 100%;
	margin: 0;
	padding: 0;
}


/* document */
body {
	font-family: "URW Gothic L", "Helvetica", "Arial", sans-serif;
	/* Background source:
	 * http://www.texturex.com/Leather-Textures/black+leather+texture+large+close+up+grain+material+dark+fabric+stock+photo.jpg.php
	 * Free offered by Free Leather Textures */
	background: url('../../../media/black-leather-texture.jpg');
}

.display {
	border: 0.2rem solid gray;
	border-radius: 0.5rem;
	background: black;
	padding: 0.5rem;
	width: 100%;
	display: block;
}
.display > output {
	display: block;
	height: 1.5rem;
	margin: 0;
	color: aqua;
}

form {
	width: 100%;
	padding: 0.5rem;
}

fieldset {
	border: none;
	margin: 0;
	font-size: 0;
}
fieldset > button {
	width: 20%;
	margin: 3.3%;
	padding: 2vw;
}
fieldset > button:nth-child(4n) {
	margin-right: 0;
}
fieldset > button:nth-child(4n+1) {
	margin-left: 0;
}

button {
	box-shadow: 0 0 0.5rem black;
	border: none;
	border-radius: 0.5rem;
	text-align: center;
	vertical-align: middle;
	margin: 0;
	font-weight: bold;
	font-size: 1.5rem;
}
output {
	font-size: 1rem;
}


button.number, button.command {
	background: white;
}
button.number:active, button.command:active {
	background: rgb(230, 230, 230);
	box-shadow: inset 0 0 0.5rem black;
}
button.operator {
	box-shadow: 0 0 0.5rem rgb(80,80, 80);
	background: black;
	color: white;
}
button.operator:active {
	background: rgb(60, 60, 60);
	box-shadow: inset 0 0 0.5rem black;
}

#key-c {
	background: red;
	color: white;
}
#key-c:active {
	background: darkred;
}
button:focus {
	box-shadow: 0 0 0.5rem deepskyblue;
}

<form>
  <fieldset class = "display">
    <output name = "output" id = "output" class = "output"></output>
    <output name = "input" id = "input" class = "input"></output>
  </fieldset>

  <fieldset>
    <button type = "button" name = "key-0" id = "key-0" class = "number" value = "0">0</button>
    <button type = "button" name = "key-1" id = "key-1" class = "number" value = "1">1</button>
    <button type = "button" name = "key-2" id = "key-2" class = "number" value = "2">2</button>
    <button type = "button" name = "key-+" id = "key-+" value = "+" class = "operator">&plus;</button>

    <button type = "button" name = "key-3" id = "key-3" class = "number" value = "3">3</button>
    <button type = "button" name = "key-4" id = "key-4" class = "number" value = "4">4</button>
    <button type = "button" name = "key-5" id = "key-5" class = "number" value = "5">5</button>
    <button type = "button" name = "key--" id = "key--" value = "-" class = "operator">&minus;</button>

    <button type = "button" name = "key-6" id = "key-6" class = "number" value = "6">6</button>
    <button type = "button" name = "key-7" id = "key-7" class = "number" value = "7">7</button>
    <button type = "button" name = "key-8" id = "key-8" class = "number" value = "8">8</button>
    <button type = "button" name = "key-*" id = "key-*" value = "*" class = "operator">&lowast;</button>

    <button type = "button" name = "key-9" id = "key-9" class = "number" value = "9">9</button>
    <button type = "button" name = "key-c" id = "key-c" class = "command">C</button>
    <button type = "button" name = "key- = " id = "key- = " class = "command">&equals;</button>
    <button type = "button" name = "key-/" id = "key-/" value = "/" class = "operator">&divide;</button>
  </fieldset>
</form>