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HTML

              
                <!--
Codepen by 101Computing.net - www.101computing.net/equalizer-animation-using-javascript
-->
<div class="equalizer">
   <span class="column" id="purple"></span>
   <span class="column" id="blue"></span>
   <span class="column" id="cyan"></span>
   <span class="column" id="green"></span>
   <span class="column" id="yellow"></span>
   <span class="column" id="orange"></span>
   <span class="column" id="red"></span> 
   <span id="black"></span>
  <br/>
<input type="button" onclick="JavaScript: play();" value="Play" id=button>
</div>
              
            
!

CSS

              
                BODY {
  background-color: black;
  margin: 20px auto 20px auto;
 }

.equalizer {
  text-align: center;
  margin: 10px auto;
  width: 380px;
}

.column {
  display: inline-block;
  width: 40px;
  margin: 5px;
}

#purple {
  height: 160px;
  background-color: #8800FF;
}

#blue {
  height: 140px;
  background-color: #0022FF;
}

#cyan {
  height: 100px;
  background-color: #22FFFF;
}

#green {
  height: 140px;
  background-color: #00BB00;
}

#yellow {
  height: 180px;
  background-color: #FFFF00;
}

#orange {
  height: 70px;
  background-color: #FF6600;
}

#red {
  height: 200px;
  background-color: #FF0000;
}

#black {
  display: inline-block;
  height: 240px;
  width: 1px;
  background-color: #000000;
}
              
            
!

JS

              
                //A function to return a random number between a min and a max value
function randomNumber(min, max) {
  number =  Math.floor((Math.random()*(max-min))+ min);
  return number;
}

//Initialise starting values
var purple, blue, cyan, green, yellow, orange, red;
purple = 160;
blue = 140;
cyan = 100;
green = 140;
yellow = 180;
orange = 70;
red = 200;

//To start with the equalizer is paused
var playing=false;

// A Function to change the height of a column more or less randomly
function changeHeight(column, height) {
  height-=randomNumber(-20,20);
  if (height>200) height=200;
  if (height<5) height=5;
  column.style.height=height + "px";  
  return height;
}


//A Function that will be run every 50ms to animate the equalizer
function animate() {
  if (playing) {
    purple = changeHeight(document.getElementById("purple"),purple);     blue = changeHeight(document.getElementById("blue"),blue); 
    cyan = changeHeight(document.getElementById("cyan"),cyan); 
    green = changeHeight(document.getElementById("green"),green); 
    yellow = changeHeight(document.getElementById("yellow"),yellow); 
    orange = changeHeight(document.getElementById("orange"),orange); 
    red = changeHeight(document.getElementById("red"),red); 
    
    //Repeat this function every 50 ms
    setTimeout(animate, 50);
  }
}

//A Function to play or pause the animation
function play() {
  if (playing) {
    playing=false;
    document.getElementById("button").value="Play";    
  } else {
    playing=true;
    document.getElementById("button").value="Pause";
    animate();
  }
}
              
            
!
999px

Console