This code snippet helps you to create a knob slider. It defines a function called main that is executed when the page is loaded. The function sets the audio volume to zero, attaches event listeners to the volume knob for mouse button clicks and releases, and creates 27 ticks.
When the mouse button is pressed, a function called onMouseDown is executed. If the audio is not already playing, it starts to play, and event listener is attached for mouse movement.
When the mouse button is released, a function called onMouseUp is executed, which removes the event listener for mouse movement.
When the mouse is moved while the button is held down, a function called onMouseMove is executed. This function computes the angle of the mouse relative to the center of the volume knob, sets the volume knob’s rotation to the angle, and updates the audio volume and tick highlights accordingly.
There are also utility functions for detecting mobile devices and for getting the appropriate event names based on whether the device is mobile or desktop.
How to Create JavaScript Knob Slider
Create the HTML structure for the knob slider as follows:
<div class="body">
<h1>Click anywhere to begin playing audio first, then drag volume knob with mouse or finger to control volume</h1>
<p>Current volume: <span id="volumeValue" class="current-value">0%</span></p>
<div class="knob-surround">
<div id="knob" class="knob"></div>
<span class="min">Min</span>
<span class="max">Max</span>
<div id="tickContainer" class="ticks"></div>
</div>
<p>Javascript written by <a href="https://www.quora.com/profile/Kevin-Lam-15">Kevin Lam</a> of <a href="https://www.ztransitions.com">zTransitions.com</a><br><br> Original HTML/CSS code forked from <a href="https://twitter.com/blucube">Ed Hicks's</a> - <a href="https://codepen.io/blucube/pen/cudAz">original HTML/CSS example</a><br><br><a href="http://dribbble.com/shots/753124-Volume-Knob">Original volume knob graphic design </a><a href="https://twitter.com/rickss">by Ricardo Salazar</a></p>
</div>
Now, style the knob slider using the following CSS styles:
@font-face {
font-family: 'Open Sans';
font-style: normal;
font-weight: 300;
src: local('Open Sans Light'), local('OpenSans-Light'), url(https://fonts.gstatic.com/s/opensans/v18/mem5YaGs126MiZpBA-UN_r8OUuhs.ttf) format('truetype');
}
@font-face {
font-family: 'Varela Round';
font-style: normal;
font-weight: 400;
src: local('Varela Round Regular'), local('VarelaRound-Regular'), url(https://fonts.gstatic.com/s/varelaround/v13/w8gdH283Tvk__Lua32TysjIfp8uK.ttf) format('truetype');
}
body {
background-color: #181818;
font-size: 100%;
font-family: "Open Sans", sans-serif;
color: #aaa;
text-align: center;
user-select: none;
}
.cd__main{
display: block !important;
}
.body{
width: 100%;
height: 100%;
background-color: black;
}
.knob-surround {
position: relative;
background-color: grey;
width: 14em;
height: 14em;
border-radius: 50%;
border: solid 0.25em #0e0e0e;
margin: 5em auto;
background: #181818;
background: -webkit-gradient(linear, left bottom, left top, color-stop(0, #1d1d1d), color-stop(1, #131313));
background: -ms-linear-gradient(bottom, #1d1d1d, #131313);
background: -moz-linear-gradient(center bottom, #1d1d1d 0%, #131313 100%);
background: -o-linear-gradient(#131313, #1d1d1d);
filter: progid:DXImageTransform.Microsoft.gradient(startColorstr='#131313', endColorstr='#1d1d1d', GradientType=0);
-webkit-box-shadow: 0 0.2em 0.1em 0.05em rgba(255, 255, 255, 0.1) inset, 0 -0.2em 0.1em 0.05em rgba(0, 0, 0, 0.5) inset, 0 0.5em 0.65em 0 rgba(0, 0, 0, 0.3);
-moz-box-shadow: 0 0.2em 0.1em 0.05em rgba(255, 255, 255, 0.1) inset, 0 -0.2em 0.1em 0.05em rgba(0, 0, 0, 0.5) inset, 0 0.5em 0.65em 0 rgba(0, 0, 0, 0.3);
box-shadow: 0 0.2em 0.1em 0.05em rgba(255, 255, 255, 0.1) inset, 0 -0.2em 0.1em 0.05em rgba(0, 0, 0, 0.5) inset, 0 0.5em 0.65em 0 rgba(0, 0, 0, 0.3);
}
.knob {
position: absolute;
width: 100%;
height: 100%;
border-radius: 50%;
-webkit-transform: rotate(0deg);
-moz-transform: rotate(0deg);
-o-transform: rotate(0deg);
-ms-transform: rotate(0deg);
transform: rotate(0deg);
z-index: 10;
}
.knob:before {
content: "";
position: absolute;
bottom: 19%;
left: 19%;
width: 3%;
height: 3%;
background-color: #a8d8f8;
border-radius: 50%;
-webkit-box-shadow: 0 0 0.4em 0 #79c3f4;
-moz-box-shadow: 0 0 0.4em 0 #79c3f4;
box-shadow: 0 0 0.4em 0 #79c3f4;
}
.min,
.max {
display: block;
font-family: "Varela Round", sans-serif;
color: rgba(255, 255, 255, 0.4);
text-transform: uppercase;
-webkit-font-smoothing: antialiased;
font-size: 70%;
position: absolute;
opacity: 0.5;
}
.min {
bottom: 1em;
left: -2.5em;
}
.max {
bottom: 1em;
right: -2.5em;
}
.tick {
position: absolute;
width: 100%;
height: 100%;
top: 0;
left: 0;
z-index: 5;
overflow: visible;
}
.tick:after {
content: "";
width: 0.08em;
height: 0.6em;
background-color: rgba(255, 255, 255, 0.2);
position: absolute;
top: -1.5em;
left: 50%;
-webkit-transition: all 180ms ease-out;
-moz-transition: all 180ms ease-out;
-o-transition: all 180ms ease-out;
transition: all 180ms ease-out;
}
.activetick:after {
background-color: #a8d8f8;
-webkit-box-shadow: 0 0 0.3em 0.08em #79c3f4;
-moz-box-shadow: 0 0 0.3em 0.08em #79c3f4;
box-shadow: 0 0 0.3em 0.08em #79c3f4;
-webkit-transition: all 50ms ease-in;
-moz-transition: all 50ms ease-in;
-o-transition: all 50ms ease-in;
transition: all 50ms ease-in;
}
h1 {
font-weight: normal;
margin: 2em 0;
}
p {
line-height: 150%;
max-width: 36em;
margin: 1em auto;
}
a {
color: #aaa;
text-decoration: none;
border-bottom: 1px solid #444;
-webkit-transition: color 0.2s ease-in;
-moz-transition: color 0.2s ease-in;
-o-transition: color 0.2s ease-in;
transition: color 0.2s ease-in;
}
a:hover,
a:focus {
color: #eee;
}
body,
.knob {
background-image: 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mGDh1KAYCtrS2t50gnRExMDAcAJSUlipUGBwfLGHj16lWVh7gVFBTINpK8r6+DgwOhTeqAzePXX3/V3Ih+/fXXivxxHFd189rp06dZDw8P1YipW7duyYTp5uZWqVVZy5YtmdTUVFJG586dGaDcOULPDjV27FhdbRMOSSNGjGDWr19vAoB79+7J+BHGoUyaNEnEG3+C+ccff9BCWqsC27ZtYwEgOjq6TP4zZswQNUTDhg01u2JMTIymIGbMmEGlpqbqav5HH31EAYCVlVWl9xUdO3Yk9aWlpVW5jWrjxo1mDTX9+vUzpKy9evUi78ncbb/66ivRviE1NZW5ffu2IiHnz58XfbxixQpScP369Wk7OzvZd9K5on///oqNNmDAgCrr+lI6eYSGhpoA4PLly/aC30RC3LRpEwMAV69eVZTBsGHDOCXnCh4hISHMoEGDKAA4efIkK+yBQqidzYsg1Dg99OvXjy4tLWX4zY8ahL1Gar52dnZmee/EZs2asQDw448/EkJ37dpFtF6YhUGKrl27am5GP/vsM0Xh/v7777r8Pn/+XLUn79ixQ8S71N0nIiJCt4dJY1wIVq9eXaEu3759e1mlbdu2NWs44s3jfIaJPXv2yASl53YkPNdQw8iRI7k5c+bYAwAftKoFKysrVWUbM2aMbBHQrFkz1TKlRk9V9O/fnzDC5+XIzMw0hYeHE0NaVlYWKezMmTOqrZ6WlkbeU8o9wkNoFAQAqSsMH06gBqUY8EOHDokMf15eXroN1K1bN1E5LVq0UKx3165dhoQ5btw48p7eGX1CQoKYvgkTJhiasJTcIW/cuEEB8j3BsmXLTMIzkydPnjAA8NdffzEjR47Uq09Wz+7du+3Onz9veFX35ZdfkjImTJjAAcDx48dF39erV88E6NuwpPDy8uICAgJEPaZRo0YVGlUUfduEk82+fftMPj4+DFA2aVWkEh61a9cWCZb3Ev/1119FRMyfP1/KnOj/n376qcrtVgkJCYZ5++abb1gAaNCggRVQ3vuvXLmiWcaUKVN0G9rCwoL0ZpJTRWos9PLyYgMCAhS7mbu7uwkAatSooSsktcAcKe7evSuq68CBA0Tb6tSpozvZLliwQJPxuLg4RTr0lvc8TCaT2cvylJQUBgCioqKsAPXNsiYsLS0ZqQv9oEGDDBEzd+5c1S67du1aYWwHq3botXXrVkN1FRYWsm+//XalDnzc3d1ZoGyFqPXe9OnTWS1N9/Pzk/Hy008/cQAwZ84cB39/f02FMhyvefnyZaGthwhq1KhRikJ78+YNIdrX15fZuHGjIqM0TVOAPOCex+bNmykASEhIYAsKClQbqGfPniYAePXqFbdu3TorAMjOzub439Q5A7y8vES0LV261KQUHwkA9evXZ0tKSkwAkJOTQ2dnZxM+u3TpotqYp06dEjViaGio4nKc58MsPH36lALKAmmMvN+4cWNRqwcHB7OCvxXLmDBhggko60F65bdv3545fPiw7L1x48YpNrIwZvHcuXOid2bPnm0JAI8fP5aV16FDB7PnUHd3d2rmzJmGJnjhVMGyrHpdlpaWiq1WXFzMOjs72wtPFpcsWSLSkg8++EC0Jl+7di15HhQURJ84cYIDgO7du8sI6NOnj6HJe8iQIUR4bm5uMuZZlqUBYMGCBZzQ54xHRZKiJScnixRJmn3IxsamasL3+B2kr6+v4WWbs7OzauUhISEsYNwkL8SlS5f+8TiLP//8kwaAgoICw3VNmTKFGz58OOFn8uTJqrLiI8/atGlTcXva0KFDVT92dXUl2si7+Rs5r1i7di27c+dOs0zrTZs25T7//HNVQfGTJQDUrVtX97z+448/ZoHyE0UjG0QhcnJyKACYPn06DQAURVWJwuzatYsFgHfeeUdbaW1sbLjVq1ezQFkoWnBwMOmWa9as4SwtLUU9Q8+D5ObNmyYAyMjIEH5HS4+HpdDK3snnYQGApUuXsgAwevRo0vAzZsxgAKBmzZqENuEJJO8vfOvWLVXa+WQHajCy5OfRtGlTDgA6duxoUgtns7CwYADJgunq1atE44YNG0YBQO3atQ1p1L59+2gACA0NVSW0U6dOplGjRulqWIsWLVgA6NOnD2dvb68VlEPKql27tkiAb731FmFs1qxZJgBITEwU9ajRo0er+lX98ssvIr6FMSQ9evTQlMnSpUtNAHTnEz7Ghke1atXKym3durVijpCkpCRN4SUnJxvqvnZ2dqKKeW1QQ35+foXGXn4jJoS/v7/qgVudOnUUh4srV65U6mzG1dWVKGVCQgL1ySefyOSUnJyszaOWkVCKly9fUkC543D37t0VtViYD/jYsWM2APD06VPujz/+UGzIadOmUXyyFjV/4qqC0cMjHpGRkYYaqV27diZ/f3+mZ8+eTFhYGAsAK1asEPVMoVO5MPB0z549VRZ6QbBp0yZDk7i3t7fahGxIUJmZmaoaxlsWZs+ercqgcOHg6upqSBAnT55khIdwdevW5f6uz6pp06ZEyCtXrjS7h6enp5fTMGjQoAppo6BFqcePH+v2sE6dOvHmfEbqraEVjaSH6OhowwKws7OjAeD8+fPML7/8YpZGbt68mSib1D2Un3O1QNM0GxoaysTHx5N92t27d5X5fvDggYipNm3aGBrCJk6caEij1bJ/CqNtpdi7dy8LlGnzzZs3qVatWlEkL6EA3377LSPMGKEEe3t7xUa7f/++jP7du3fTQJnHTZMmTUxjxoxhhck89fYv7dq10x0leOu14iKIzzUbHh7OHDt2rEKa+vTpU/qzzz5TOigSCeKLL75ghUmG1eDg4CAa0oS5sp49e8YCQHx8PCc9g7hz546iMM6cOVNpp4ewsDDNHiVMGMofPUt5/eWXX2QrO2nSNhGaNm1aoYll1qxZXGxsLCn4yJEjIsH4+/ubAGDZsmWqPW/s2LGKmifV4oULF1qmp6fr9mBNRgG8fv2aGT16NAWURfPyv5uzx9DD1KlTZXSqRTATqHllAOUhBEKMHz++wrtV/mqJIUOGqDb82bNnuXXr1hHNiouLU9Xu7777rlLC09qgWltbc7du3WLUopKN4MaNGywArFmzxgSoZ9VTxWeffaZaeXR0tKo3n1paJyn0YiVOnz7N3rx5kwYArU2hEaSmptLCVH9VgYMHD1J79uwx24g4bdo0VSX28vJSp7F69epsREQE/fDhQ9UC+ET6Tk5O1Llz5xQLMzJPAEBWVpah9968eVPla3ShE4YUPXr0oF+9ekXPnz9fkT8XFxdD0VMrVqyQ1dG2bVtV2WZnZ4vfVzr9AsQCzsrK4jiOI4S6uLio9g4bGxtDPYc3b/CQpvV49eoV27p1a7p3796Gh8uIiAhu4sSJmg0eHh5uAoxbfqWrUaA8fxYAXL9+nQPKzla6detGeJfaAAH9RYIm1HyrOnfuTLz+Hj58SAjgfW4TEhJEhCQlJdGAsesjeLz11ls0AAwaNEimrVpXafj5+bHDhw8XPZcG2fBYsGCBrGw1C/jp06d1Bfn8+XPVYai4uFhVSdTMOQSJiYkihnJzc0UfKJ1f8LvOwMBABgCCgoIslbLvfPPNNzYAkJGRwTo5ObGvX7/mDh06xAFAu3btOGE67t69e1OHDh2iACAvL0/GbFhYmKaGN2nSRPQ8JSVFNB8Kg5GUIHUxFeL333+Xaf/3339PaNy5c6dIZlqppi5fvlxeFm/Zzc7O1pwIHR0dSQsPHTpUxsjUqVMJ8f369TNr/b9+/XrDE2bbtm0tASAxMVFUR+vWrSnhXSSA2FQiBH+yKIQw7SsAVKtWjRsyZAi9d+9eRUNlx44dLQFxrmAlCJNn6mLOnDkywc2dO9cEGJ8HeAitneZAmupJCN7sUlHw96GMGzdOcTK+fv264lAxadIkqylTpgg3peTcgk8c+sEHHxBB9+rVi+3Tp4/ZtDo7O4u/4S2TSrC2tqZdXFwUCeY3VlIo3WojxPz58xnAuFumElauXElnZWWpnoMIYTQ8T4jly5cbWjLzCRSEePnyJfvmzZt/L6pYz81GiL179zJAeaALUJ7Ia8uWLYrl2NnZKQr2xIkTZu0rhHdkffDBB7JvX79+TTTT399fVbNfvnyp+EwpNn3kyJEyS/bIkSMJP0pZJEQQmjP4rGhbtmxR1XA+xR0AXLlyxdCYbzTUa9u2barljRs3jgOAgoICs4aEK1euMEBZvnYPDw+ze0ppaamhb6QXkv2rUNLsiIgIE2+76devn6YDgjRniBYuXbrExMTEiN7/6KOPOD4+MDQ0lAL0L5sBgPz8fLZu3bqaSsT7HX/44YeVcmjgL97k/QqE6NKli1LZxmTyzjvvaDLAe5brwVxTiHRVIrzC1Ai8vb1ldOvdkCYMEKos7OzsqI8++sgEyH2Ppelrr1+/br5PF5+/UM0FlE+NpARhljXe0UF43Z5RCK/JMwd6ztLHjh1TtVAEBQXJvtXK5cjj559/pgEgMDCQBoBFixbpRZfZDx061JjS8tZXb29vQ+O3cC+iBE9PT11N7Nq1q2JdwiuRhg8fzh48eFDE6JkzZ/6xsbxbt26qZUdFRcmeBQYGGupxffr0MQHikD9duLi4KHYrYax3q1atFAsUXo3EXwh84MAB1cqfPXvGNmjQQPQ8LCzMVFhYSBgsKSlR1DjecxIoDyp1dnY2SRcgdnZ2hKZHjx5Veok6evRoprCwUMbT4sWLWaUUU1euXGGENxjJcPToUWbmzJmaLXrt2rUKEb569WpamPwSKEuvKn1Py1Cphbt375oAQMnlRgnW1tYyQfCJ2QDAzc1NkQ5phjoj8PX1/XdWX/xtZEbh5+dHA/IrWMPCwkg5TZo0EQlCGoImBG/wnD9/foXvnuIRGRlp1lLaxsZG8f3s7GzVcq5fv04vWrRI9lwYp24YLi4uooL4TVReXp7p8ePHhjQgKChIpnUVvekgNjZWpAxCHzBzNq0AEBYWptgb+DzE27dv1yxvw4YNhnu1g4MDxZukpBA5lgQFBXGA+m0Ias4DeuDHeSUn5YSEBBlh0riS5ORkVWb5YJnDhw9XekgQnmlooUuXLmbJQXpP+/fff88AZRc3A+JkC3//X/Z+cHAws3XrVsrDw4NLT09X7HatWrX6x0MFtLB+/XrNYaWoqIgbMmSIqplddIuNBEIPQqVU4FqwtbUl78fGxirKaPXq1RxQnhtGOFf6+flRs2bNKleIzMxMJjc3l/wQFBQksoguWbLEEij3GudhY2Pzj05Wtra2/68KIEVqaqpVfHw8l5eXp9lgGzdutJPmgOe9KNW8KT09PSkAaNq0qVjppJcUHz58mMnNzVV1bpBm4+FXFHXq1CHC3LFjh0yzpTcz68ZJSCDN0R4QEKB4VpGfn1/hqKbt27dzUtsXvxRPSEgg9c+cOdNwHUrWAxk2b95MKhVevihFx44dK3UmwacBNBerVq1iAKBOnTqEthMnThDG+PAFHrNnz2YAgGEYFgBiYmJ0E/4DgLu7u4h3YfyIWsZRI7h48WLV+XwNHDiQkXpCGLm4BADef/99kTZEREQQLd63b5+ipoSGhlIjR44kglA60ePRsmVLkfbyVl2g7FhY8ozUN3bsWLMUa+3atYpDp1JKD6AspWF8fLym4PnsFzz2799fseG/ZcuWnNCeNGrUKEPMFRUVUZMmTdJ9Nycnx/T555/TsbGxXExMjOpQOW3aNJPaRZFKSElJMUvDz549a2j+un37tmK50nxdiYmJrPRY+c2bN1xmZiZpOBcXF5u7d++KeXr58qVul7p48aKM2DVr1si+4xNd9ujRgxE2hp+fn6Y2NG7cWLXhhJqXnp6uS6s0vyEfS//XX3+ROvgEBLyDhRRS/2Qp0tLSuIyMDDokJESYW8WsuWvJkiX0sGHDLJVyjCmipKREVcOklQtPBtWW0EIII1pTUlLs1ZL6q50iKkErU6pWQ1pbW1OhoaGsNNFaeHi48FCO+Zse0oBr1qwh9dna2rIA8Oeff1ZsGBLacIDyRDDCMGAAaAVaj5QAAALLSURBVNKkiW63/+abb1S7Op+k3lycPXvWBAAlJSWq4WlKUNrERUVFUQDw4sULUYNdvHhR1kjCmJWvv/5acfg8fvy4WbfBVQqDBg2SCfDRo0fM8OHDWX7Zd+TIEUVtMxIydvbsWRoou/rIKE3Cy5N593+liFn+PsbQ0FBRI+7fv19RKaZOnVrpcAVzwDAMO378ePXhid/sFBQU6I5hWv5FaWlpVZPNQAdqGasBICAggD58+LDicyP3CUohzJKXmJioqjzTp09XrNNoAgEjIRYE/OUqHh4eZmuSr68vOcrkcfHiRZO1tbUJAFauXEm0mD9lk+L48eM0UH53IL80NmdTZg7mzJnDAeUTfmhoKFNUVER4p2n633NoOHLkiGZlf/zxB62Ww116arZy5UqO95WiaZpo0MuXLzkAaNCgAflNLQGk9AJHo6AoSvW7J0+e0Hz0MH8He6NGjUjvEd45wqNFixYcfwNOtWrVZLTWq1dPphzR0dEsULbPELrdOjo6qplQjPHKZwS1tbU1AWW3qBn6EICPjw8hZOfOnVRwcLCsl/HJ6s118VECx3Gqw5k0ZZRapjo+ZsPe3p4IWXhaqYS+ffuy5twVMm/ePHbXrl2ULM4yKipKJISUlBQ6NjaW4TPuqDkACDF48GBDhKhFI5HUdhWEWk6Vli1bmj1nCG+Zk0Lt1uv79+8bop/PnaKGunXrlpUjzIdev359GXN8Wj8AOHXqlKHe8eDBAw4oiyc5fvy4LsEff/yxMNJVtesmJiZyDx48qNAwVpnway3wi5kbN24wBQUFNkB5ggUt9O7dm2ndujUjuxdReG13dHS0rKCjR48yfCz28OHDGaD8gnclt6DmzZvLynj27Bl94cKFKgsxe++994i2ZmZmajaQWgOqXYaWl5dH169fnwOAqVOnkndOnz5NeOXzfgmhdy2e0h1YALB48WKGz6/yf8RxFhxL0FjRAAAAAElFTkSuQmCC);
}
Finally, add the following JavaScript function for the functionality:
var knobPositionX;
var knobPositionY;
var mouseX;
var mouseY;
var knobCenterX;
var knobCenterY;
var adjacentSide;
var oppositeSide;
var currentRadiansAngle;
var getRadiansInDegrees;
var finalAngleInDegrees;
var volumeSetting;
var tickHighlightPosition;
var audio = new Audio("https://www.cineblueone.com/maskWall/audio/skylar.mp3"); //Celine Dion's "Ashes"
var startingTickAngle = -135;
var tickContainer = document.getElementById("tickContainer");
var volumeKnob = document.getElementById("knob");
var boundingRectangle = volumeKnob.getBoundingClientRect(); //get rectangular geometric data of knob (x, y, width, height)
function main()
{
audio.volume = 0; //start at zero volume
volumeKnob.addEventListener(getMouseDown(), onMouseDown); //listen for mouse button click
document.addEventListener(getMouseUp(), onMouseUp); //listen for mouse button release
createTicks(27, 0);
}
//on mouse button down
function onMouseDown()
{
//start audio if not already playing
if(audio.paused == true)
{
//mobile users must tap anywhere to start audio
//https://developers.google.com/web/updates/2017/09/autoplay-policy-changes
var promise = audio.play();
if (promise !== undefined) {
promise.then(_ => {
audio.play();
}).catch(error => {
});
}
}
document.addEventListener(getMouseMove(), onMouseMove); //start drag
}
//on mouse button release
function onMouseUp()
{
document.removeEventListener(getMouseMove(), onMouseMove); //stop drag
}
//compute mouse angle relative to center of volume knob
//For clarification, see my basic trig explanation at:
//https://www.quora.com/What-is-the-significance-of-the-number-pi-to-the-universe/answer/Kevin-Lam-15
function onMouseMove(event)
{
knobPositionX = boundingRectangle.left; //get knob's global x position
knobPositionY = boundingRectangle.top; //get knob's global y position
if(detectMobile() == "desktop")
{
mouseX = event.pageX; //get mouse's x global position
mouseY = event.pageY; //get mouse's y global position
} else {
mouseX = event.touches[0].pageX; //get finger's x global position
mouseY = event.touches[0].pageY; //get finger's y global position
}
knobCenterX = boundingRectangle.width / 2 + knobPositionX; //get global horizontal center position of knob relative to mouse position
knobCenterY = boundingRectangle.height / 2 + knobPositionY; //get global vertical center position of knob relative to mouse position
adjacentSide = knobCenterX - mouseX; //compute adjacent value of imaginary right angle triangle
oppositeSide = knobCenterY - mouseY; //compute opposite value of imaginary right angle triangle
//arc-tangent function returns circular angle in radians
//use atan2() instead of atan() because atan() returns only 180 degree max (PI radians) but atan2() returns four quadrant's 360 degree max (2PI radians)
currentRadiansAngle = Math.atan2(adjacentSide, oppositeSide);
getRadiansInDegrees = currentRadiansAngle * 180 / Math.PI; //convert radians into degrees
finalAngleInDegrees = -(getRadiansInDegrees - 135); //knob is already starting at -135 degrees due to visual design so 135 degrees needs to be subtracted to compensate for the angle offset, negative value represents clockwise direction
//only allow rotate if greater than zero degrees or lesser than 270 degrees
if(finalAngleInDegrees >= 0 && finalAngleInDegrees <= 270)
{
volumeKnob.style.transform = "rotate(" + finalAngleInDegrees + "deg)"; //use dynamic CSS transform to rotate volume knob
//270 degrees maximum freedom of rotation / 100% volume = 1% of volume difference per 2.7 degrees of rotation
volumeSetting = Math.floor(finalAngleInDegrees / (270 / 100));
tickHighlightPosition = Math.round((volumeSetting * 2.7) / 10); //interpolate how many ticks need to be highlighted
createTicks(27, tickHighlightPosition); //highlight ticks
audio.volume = volumeSetting / 100; //set audio volume
document.getElementById("volumeValue").innerHTML = volumeSetting + "%"; //update volume text
}
}
//dynamically create volume knob "ticks"
function createTicks(numTicks, highlightNumTicks)
{
//reset first by deleting all existing ticks
while(tickContainer.firstChild)
{
tickContainer.removeChild(tickContainer.firstChild);
}
//create ticks
for(var i=0;i<numTicks;i++)
{
var tick = document.createElement("div");
//highlight only the appropriate ticks using dynamic CSS
if(i < highlightNumTicks)
{
tick.className = "tick activetick";
} else {
tick.className = "tick";
}
tickContainer.appendChild(tick);
tick.style.transform = "rotate(" + startingTickAngle + "deg)";
startingTickAngle += 10;
}
startingTickAngle = -135; //reset
}
//detect for mobile devices from https://www.sitepoint.com/navigator-useragent-mobiles-including-ipad/
function detectMobile()
{
var result = (navigator.userAgent.match(/(iphone)|(ipod)|(ipad)|(android)|(blackberry)|(windows phone)|(symbian)/i));
if(result !== null)
{
return "mobile";
} else {
return "desktop";
}
}
function getMouseDown()
{
if(detectMobile() == "desktop")
{
return "mousedown";
} else {
return "touchstart";
}
}
function getMouseUp()
{
if(detectMobile() == "desktop")
{
return "mouseup";
} else {
return "touchend";
}
}
function getMouseMove()
{
if(detectMobile() == "desktop")
{
return "mousemove";
} else {
return "touchmove";
}
}
main();
That’s all! hopefully, you have successfully created the JavaScript knob slider. If you have any questions or suggestions, feel free to comment below.
