HTML Canvas Eraser and Pen not working as intended

March 30, 2023

I have a canvas that has a pen and eraser button. Unfortunately, clicking either of the buttons does nothing. I want the eraser to change into the color white and the pen to be black. I cannot figure out what is causing my code to not work. Here is my javascript code.

var mousePressed = false;
var lastX, lastY;
var ctx = document.getElementById('myCanvas').getContext("2d");

function InitThis() {
    $('#myCanvas').mousedown(function (e) {
        mousePressed = true;
        Draw(e.pageX - $(this).offset().left, e.pageY - $(this).offset().top, false);
    });

    $('#myCanvas').mousemove(function (e) {
        if (mousePressed) {
            Draw(e.pageX - $(this).offset().left, e.pageY - $(this).offset().top, true);
        }
    });

    $('#myCanvas').mouseup(function (e) {
        if (mousePressed) {
            mousePressed = false;
            cPush();
        }
    });

    $('#myCanvas').mouseleave(function (e) {
        if (mousePressed) {
            mousePressed = false;
            cPush();
        }
    });
    
    $("#pen").click(function() {
        ctx.strokeStyle = "#000000";
    });

    $("#erase").click(function() {
        ctx.strokeStyle = "#FFFFFF";
    });
    
    var canvas = document.getElementById('myCanvas');
    var canvasOffsetX = canvas.offsetLeft;
    var canvasOffsetY = canvas.offsetTop;

    canvas.width = window.innerWidth - canvasOffsetX;
    canvas.height = window.innerHeight - canvasOffsetY;
    
    drawImage();
}

function drawImage() {
    var image = new Image();
    image.src = 'transparent.jpg';
    $(image).load(function () {
        ctx.drawImage(image, 0, 0);
        cPush();
    });    
}

function Draw(x, y, isDown) {
    if (isDown) {
        ctx.beginPath();
        ctx.strokeStyle = $('#selColor').val();
        ctx.lineWidth = $('#selWidth').val();
        ctx.lineJoin = "round";
        ctx.moveTo(lastX, lastY);
        ctx.lineTo(x, y);
        ctx.closePath();
        ctx.stroke();
    }
    lastX = x;
    lastY = y;
}

var cPushArray = new Array();
var cStep = -1;

function cPush() {
    cStep++;
    if (cStep < cPushArray.length) { cPushArray.length = cStep; }
    cPushArray.push(document.getElementById('myCanvas').toDataURL());
}

function cUndo() {
    if (cStep > 0) {
        cStep--;
        var canvasPic = new Image();
        canvasPic.src = cPushArray[cStep];
        canvasPic.onload = function () { ctx.drawImage(canvasPic, 0, 0); }
    }
}

function cRedo() {
    if (cStep < cPushArray.length-1) {
        cStep++;
        var canvasPic = new Image();
        canvasPic.src = cPushArray[cStep];
        canvasPic.onload = function () { ctx.drawImage(canvasPic, 0, 0); }
    }
}

// Set up touch events for mobile, etc
const canvas = document.getElementById('myCanvas');
canvas.addEventListener("touchstart", function (e) {
        mousePos = getTouchPos(canvas, e);
  var touch = e.touches[0];
  var mouseEvent = new MouseEvent("mousedown", {
    clientX: touch.clientX,
    clientY: touch.clientY
  });
  canvas.dispatchEvent(mouseEvent);
}, false);
canvas.addEventListener("touchend", function (e) {
  var mouseEvent = new MouseEvent("mouseup", {});
  canvas.dispatchEvent(mouseEvent);
}, false);
canvas.addEventListener("touchmove", function (e) {
  var touch = e.touches[0];
  var mouseEvent = new MouseEvent("mousemove", {
    clientX: touch.clientX,
    clientY: touch.clientY
  });
  canvas.dispatchEvent(mouseEvent);
}, false);

// Get the position of a touch relative to the canvas
function getTouchPos(canvasDom, touchEvent) {
  var rect = canvasDom.getBoundingClientRect();
  return {
    x: touchEvent.touches[0].clientX - rect.left,
    y: touchEvent.touches[0].clientY - rect.top
  };
}

// Prevent scrolling when touching the canvas
document.body.addEventListener("touchstart", function (e) {
  if (e.target == canvas) {
    e.preventDefault();
  }
}, false);
document.body.addEventListener("touchend", function (e) {
  if (e.target == canvas) {
    e.preventDefault();
  }
}, false);
document.body.addEventListener("touchmove", function (e) {
  if (e.target == canvas) {
    e.preventDefault();
  }
}, false);

function save(){
    var canvas = document.querySelector("#myCanvas");
    var image = canvas.toDataURL("image/jpg").replace("image/jpg", "image/octet-stream");

    var element = document.createElement('a');
    var filename = 'drawing.jpg';
    element.setAttribute('href', image);
    element.setAttribute('download', filename);
    element.click();
}

var hea der = document.getElementById("myDIV");
var btns = header.getElementsByClassName("btn");
for (var i = 0; i < btns.length; i++) {
  btns[i].addEventListener("click", function() {
  var current = document.getElementsByClassName("active");
  current[0].className = current[0].className.replace(" active", "");
  this.className += " active";
  });
}

Here is a codepen with the entire code. As you can see, the pen and eraser buttons do nothing. https://codepen.io/adilene/pen/zYJQYXq

>Solution :

I am not a canvas guy, but looking into the code, found that
ctx.strokeStyle = $('#selColor').val(); this is causing issue. Commenting this line worked for me.

var mousePressed = false;
var lastX, lastY;
var ctx = document.getElementById('myCanvas').getContext("2d");
var stroke = "#000000";

function InitThis() {
    $('#myCanvas').mousedown(function (e) {
        mousePressed = true;
        Draw(e.pageX - $(this).offset().left, e.pageY - $(this).offset().top, false);
    });

    $('#myCanvas').mousemove(function (e) {
        if (mousePressed) {
            Draw(e.pageX - $(this).offset().left, e.pageY - $(this).offset().top, true);
        }
    });

    $('#myCanvas').mouseup(function (e) {
        if (mousePressed) {
            mousePressed = false;
            cPush();
        }
    });

    $('#myCanvas').mouseleave(function (e) {
        if (mousePressed) {
            mousePressed = false;
            cPush();
        }
    });
    
    $("#pen").click(function() {
        ctx.strokeStyle = stroke;
    });

    $("#erase").click(function() {
        stroke = ctx.strokeStyle;
        ctx.strokeStyle = "#FFFFFF";
    });
    
    var canvas = document.getElementById('myCanvas');
    var canvasOffsetX = canvas.offsetLeft;
    var canvasOffsetY = canvas.offsetTop;

    canvas.width = window.innerWidth - canvasOffsetX;
    canvas.height = window.innerHeight - canvasOffsetY;
    
    drawImage();
}

function drawImage() {
    var image = new Image();
    image.src = 'transparent.jpg';
    $(image).load(function () {
        ctx.drawImage(image, 0, 0);
        cPush();
    });    
}

function Draw(x, y, isDown) {
    if (isDown) {
        ctx.beginPath();
        // Changing this line worked
        //ctx.strokeStyle = $('#selColor').val();
        ctx.lineWidth = $('#selWidth').val();
        ctx.lineJoin = "round";
        ctx.moveTo(lastX, lastY);
        ctx.lineTo(x, y);
        ctx.closePath();
        ctx.stroke();
    }
    lastX = x;
    lastY = y;
}

var cPushArray = new Array();
var cStep = -1;

function cPush() {
    cStep++;
    if (cStep < cPushArray.length) { cPushArray.length = cStep; }
    cPushArray.push(document.getElementById('myCanvas').toDataURL());
}

function cUndo() {
    if (cStep > 0) {
        cStep--;
        var canvasPic = new Image();
        canvasPic.src = cPushArray[cStep];
        canvasPic.onload = function () { ctx.drawImage(canvasPic, 0, 0); }
    }
}

function cRedo() {
    if (cStep < cPushArray.length-1) {
        cStep++;
        var canvasPic = new Image();
        canvasPic.src = cPushArray[cStep];
        canvasPic.onload = function () { ctx.drawImage(canvasPic, 0, 0); }
    }
}

// Set up touch events for mobile, etc
const canvas = document.getElementById('myCanvas');
canvas.addEventListener("touchstart", function (e) {
        mousePos = getTouchPos(canvas, e);
  var touch = e.touches[0];
  var mouseEvent = new MouseEvent("mousedown", {
    clientX: touch.clientX,
    clientY: touch.clientY
  });
  canvas.dispatchEvent(mouseEvent);
}, false);
canvas.addEventListener("touchend", function (e) {
  var mouseEvent = new MouseEvent("mouseup", {});
  canvas.dispatchEvent(mouseEvent);
}, false);
canvas.addEventListener("touchmove", function (e) {
  var touch = e.touches[0];
  var mouseEvent = new MouseEvent("mousemove", {
    clientX: touch.clientX,
    clientY: touch.clientY
  });
  canvas.dispatchEvent(mouseEvent);
}, false);

// Get the position of a touch relative to the canvas
function getTouchPos(canvasDom, touchEvent) {
  var rect = canvasDom.getBoundingClientRect();
  return {
    x: touchEvent.touches[0].clientX - rect.left,
    y: touchEvent.touches[0].clientY - rect.top
  };
}

// Prevent scrolling when touching the canvas
document.body.addEventListener("touchstart", function (e) {
  if (e.target == canvas) {
    e.preventDefault();
  }
}, false);
document.body.addEventListener("touchend", function (e) {
  if (e.target == canvas) {
    e.preventDefault();
  }
}, false);
document.body.addEventListener("touchmove", function (e) {
  if (e.target == canvas) {
    e.preventDefault();
  }
}, false);

function save(){
    var canvas = document.querySelector("#myCanvas");
    var image = canvas.toDataURL("image/jpg").replace("image/jpg", "image/octet-stream");

    var element = document.createElement('a');
    var filename = 'drawing.jpg';
    element.setAttribute('href', image);
    element.setAttribute('download', filename);
    element.click();
}

var header = document.getElementById("myDIV");
var btns = header.getElementsByClassName("btn");
for (var i = 0; i < btns.length; i++) {
  btns[i].addEventListener("click", function() {
  var current = document.getElementsByClassName("active");
  current[0].className = current[0].className.replace(" active", "");
  this.className += " active";
  });
}

:root {
    --b: white; /* container bgs */
    --bw: 3px; /* border width */
    --bc: #34f2ff; /* border color */
    --bcc: #34f2ff; /* border color outset */
    --h: #dbfcff; /* header bgs */
    --pink: #e74492;
    --bt: solid;
    --color:#000;
}

* {
    box-sizing: border-box;
    scrollbar-color:#e74492 #fff;
    scrollbar-width:thin;
    cursor:url('https://files.catbox.moe/h0vf27.png'), pointer;
}

::-webkit-scrollbar{
    width: 10px;
    height: 10px;
}

::-webkit-scrollbar-thumb{
    background: #e74492;
    border: 3.5px var(--bt) #FFFFFF;
    border-radius: 0px;
}

::-webkit-scrollbar-thumb:hover{
    background: #ed75ae;
}

::-webkit-scrollbar-track{
    background: #FFFFFF;
    border-radius: 0px;
    box-shadow: inset 0px 0px 0px 0px #F0F0F0;
}

::selection {
    background: #34f2ff;
    color:white;
}
 
::-moz-selection {
    background: #34f2ff;
    color:white;
}

a{
    text-decoration: none;
    color:#e74492;
    text-shadow: 2px 2px 1px #34f2ff;
    transition:0.3s;
}

a:hover{
    
    transition:0.3s;
    letter-spacing:1.5px;
}

@font-face{
    font-family:'Pixel Operator';
    src:url(https://files.catbox.moe/kyguk9.ttf);
}

@font-face{
    font-family:'Orbitron';
    src:url(https://files.catbox.moe/cckohn.ttf);
}

body{
    background-color:var(--bg);
    text-align:center;
    margin:0px;
    padding:0;
    overflow:hidden;
    font-family: 'Pixel Operator';
    font-size:20px;
    animation:fadeEffect 0.5s;
    height:100%;
}

@keyframes fadeEffect {
    from {opacity: 0;}
    to {opacity: 1;}
}

p{
    line-height:100%;
}

a{
    text-decoration: none;
    color:#e74492;
    text-shadow: 2px 2px 1px #34f2ff;
    transition:0.3s;
}

a:hover{
    letter-spacing: 2px;
}

header{
    color:#e74492;
    font-family: 'Orbitron';
    font-size:20px;
}

ul{
    list-style:none;
}

ul li:before{
    content:"\25A0 ";
    color:#e74492;
    display:inline-block;
    width:1em;
    margin-left:-1.3em;
}

img {
    user-drag: none;
    -webkit-user-drag: none;
    user-select: none;
    -moz-user-select: none;
    -webkit-user-select: none;
    -ms-user-select: none;
}

button{
    border: var(--bw) var(--bt) var(--bc);
    outline: none;
    padding: 5px 8px;
    background-color: #dbfcff;
    font-family: 'Pixel Operator';
    font-size:16px;
    margin:2px 0px;
    transition:0.2s;
    color:#e74492;
}

button:hover, .active{
    background-color: #e74492;
    color:white;
    transition:0.2s;
}

.btn:hover, .active{
    background-color: #e74492;
    color:white;
    transition:0.2s;
}

#container {
    height: 100vh;
    display: flex;
}

#drawing-board{
    background-color:white;
}

#toolbar {
    display:flex;
    flex-direction: column;
    padding: 10px;
    width: 120px;
    text-align:center;
    background-color: var(--h);
    border:var(--bw) var(--bt) var(--bc);
}

#toolbar * {
    margin-bottom:10px;
}

#toolbar input {
    width: 100%;
    border:var(--bw) var(--bt) var(--bc);
}

#toolbar input{
    border:var(--bw) var(--bt) var(--bc);
    background-color:white;
}

canvas {
    touch-action: none;
}

.thumbnail img{
    width:100%;
    height: 200px;
    object-fit: cover;
    box-sizing: border-box;
    padding:10px;
    background-color:#dbfcff;
    border: var(--bw) var(--bt) var(--bc);
    display:block;
}

.thumbnail img:hover{
    opacity:0.8;
    transition:0.3s;
}

.wrap{
    width:100%;
    margin:auto;
    gap:10px;
    display: flex;
    flex-wrap:wrap;
    justify-content: center;
}

.wrap div{
    width:calc(24.8% - 5px);
}

.wrap div div{
    width:100%;
}

input::file-selector-button {
    border: var(--bw) var(--bt) var(--bc);
    outline: none;
    padding: 5px 8px;
    background-color: #dbfcff;
    font-family: 'Pixel Operator';
    font-size:16px;
    margin:2px 10px;
    transition:0.2s;
    color:#e74492;
}

 input::file-selector-button:hover {
    background-color: #e74492;
    color:white;
    transition:0.2s;
    cursor:url('https://files.catbox.moe/h0vf27.png'), pointer;
}

#name input, #url input{
    border:var(--bw) var(--bt) var(--bc);
    background-color:white;
}

#myFile{
    font-family:'Pixel Operator';
    font-size:20px;
}

.desc{
    padding:5px 10px;
    border-left:var(--bw) var(--bt) var(--bc);
    border-bottom:var(--bw) var(--bt) var(--bc);
    border-right:var(--bw) var(--bt) var(--bc);
    box-sizing:border-box;
    background-color:white;
        }

<script src="//ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js" type="text/javascript"></script>
    <style>
        button{
            width:100%;
        }
    </style>
    <body onload="InitThis();">
        <section id="container">
            <div id="toolbar">
                <header>OEKAKI</header>
                <label for="selColor">Pen Color</label>
                <input id="selColor" name="selColor" type="color">
                <label for="selWidth">Line Width</label>
                <input id="selWidth" name="selWidth" type="number" value="3">
                <div style="margin-bottom:0px;" id="myDIV">
                    <button id="pen" class="btn active">Pen</button>
                    <button id="erase" class="btn">Eraser</button>
                </div>
                <button onclick="javascript:cUndo();return false;">Undo</button>
                <button onclick="javascript:cRedo();return false;">Redo</button>
                <button class="download" onclick="save()">Save</button>
                <div>Feel free to upload your images to the gallery for a chance to be featured!</div>
                <a href="/gallery.html"><button style="margin-bottom:0px;">Gallery</button></a>
                <button onclick="javascript:drawImage();return false;">Clear</button>
            </div>
            <div class="drawing-board">
                <canvas id="myCanvas"></canvas>
            </div>
        </section>
        
        <script src="script.js"></script>
    </body>