JavaScriptでタイムラインのステップを制御するボタンを作成する方法

const timeline = document.getElementById('timeline1');
const btnChangeStyle = document.getElementById('toggle-style');
const btnChangeLabelPos = document.getElementById('toggle-label-pos');
const steps = timeline.querySelectorAll('.timeline__step');

function changeStyle() {
  timeline.classList.toggle('timeline--points');
}

function changeLabelsPosition() {
  timeline.classList.toggle('timeline--labels-underline');
}

btnChangeStyle.addEventListener('click', changeStyle);
btnChangeLabelPos.addEventListener('click', changeLabelsPosition);


document.querySelector('#next').addEventListener('click', () => {
  let steps = document.querySelectorAll("#timeline1 > .timeline__step");
  let passedSteps = document.querySelectorAll("#timeline1 > .timeline__step--passed");
  let firstStep = document.querySelector("#timeline1 > .timeline__step:first-child");
  let activeStep = document.querySelector("#timeline1 > .timeline__step--active");
  let nextStep;
  if(steps.length == passedSteps.length) {
    return;
  }
  if(activeStep) {
    activeStep.classList.remove('timeline__step--active');
    activeStep.classList.add('timeline__step--passed');
    nextStep = activeStep.nextElementSibling
  }
  if(nextStep) {
    nextStep.classList.add('timeline__step--active'); 
  } else if(passedSteps.length === 0) {
    firstStep.classList.add('timeline__step--active'); 
  }
});


document.querySelector('#prev').addEventListener('click', () => {
  let passedSteps = document.querySelectorAll("#timeline1 > .timeline__step--passed");
  let lastStep = document.querySelector("#timeline1 > .timeline__step:last-child");
  let activeStep = document.querySelector("#timeline1 > .timeline__step--active");
  let prevStep;
  if(passedSteps.length === 0) {
    return;
  }
  if(activeStep) {
    activeStep.classList.remove('timeline__step--active');
    activeStep.classList.remove('timeline__step--passed');
    prevStep = activeStep.previousElementSibling;
  }
  if(prevStep) {
    prevStep.classList.add('timeline__step--active'); 
  } else if(passedSteps.length === steps.length) {
    lastStep.classList.remove('timeline__step--passed');
    lastStep.classList.add('timeline__step--active');
  }
});

body {
  margin-top: 50px;
}

/* Timeline */

.timeline {
  position: relative;
  display: flex;
  justify-content: space-between;
  align-items: center;
  width: 100%;
  height: 30px;
  margin-bottom: 50px; /* optional property */
}

.timeline__step {
  position: relative;
  text-align: center;
  flex-grow: 1;
  height: 2px;
  background-color: lightgrey;
}

.timeline__step::before,
.timeline__step:last-child::after {
  content: "";
  position: absolute;
  top: 50%;
  left: 0;
  display: block;
  height: 11px;
  width: 11px;
  background-color: #ffffff;
  border: 2px solid lightgray;
  border-radius: 50%;
  box-sizing: border-box;
  box-shadow: 0 0 0 2px #fff;
  transform: translate(-50%, -50%);
}

.timeline__step:first-child::before {
  transform: translate(0%, -50%);
}

.timeline__step:last-child::after {
  left: auto;
  right: 0;
  transform: translate(0%, -50%);
}

.timeline__label {
  position: absolute;
  left: 50%;
  bottom: 10px;
  transform: translateX(-50%);
}

.timeline__step--passed,
.timeline__step--passed::before,
.timeline__step--passed:last-child::after {
  background-color: green;
  border-color: green;
}

.timeline__step--active,
.timeline__step--active::before {
  background-color: orange;
  border-color: orange;
}

/* Timeline points style (SASS Syntax) */

.timeline--points {
  
  .timeline__step:first-child {
   
    .timeline__label {
      transform: translateX(0);
    }
  }
  
  .timeline__step:last-child {
    position: absolute;
    right: 0;
    width: 100%;
    background: none;
    
    &::before {
      left: auto;
      right: 0;
      transform: translate(0, -50%);
    }
    
    &::after {
      display: none;
    }
    
    .timeline__label {
      left: auto;
      right: 0;
      transform: translateX(0);
    }
  }
  
  .timeline__label {
    position: absolute;
    left: 0;
    transform: translateX(-50%);
  }
  
  .timeline__step--active {
    background: lightgray;
  }
  
}

/* Timeline label position (SASS Syntax) */

.timeline--labels-underline {
   
  .timeline__label {
    bottom: auto;
    top: 10px;
  }
}

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <link href="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/4.1.3/css/bootstrap.min.css" rel="stylesheet"/>
  <title>Document</title>
</head>
<body>
  <div class="container">
    
    <div class="timeline" id="timeline1">
      <div class="timeline__step timeline__step--passed">
        <span class="timeline__label">Step 1</span>
      </div>
      <div class="timeline__step timeline__step--passed">
        <span class="timeline__label">Step 2</span>
      </div>
      <div class="timeline__step timeline__step--active">
        <span class="timeline__label">Step 3</span>
      </div>
      <div class="timeline__step">
        <span class="timeline__label">Step 4</span>
      </div>
      <div class="timeline__step">
        <span class="timeline__label">Step 5</span>
      </div>
    </div>
    
    <button id="toggle-style" type="button" class="btn btn-outline-primary">Toggle style</button>
    <button id="toggle-label-pos" type="button" class="btn btn-outline-primary">Toggle label position</button>
    
    <button id="prev" class="btn btn-primary">&lt Prev</button>
    <button id="next" class="btn btn-primary">Next &gt</button>
    
  </div>
  
  
</body>
</html>