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HTML

              
                <h1 id="el">
  Text 1
</h1>
<button type="button" onclick="change_to_blue()">Change to Blue</button>
<button type="button" onclick="change_to_green()">Change to Green</button>
<button type="button" onclick="change_to_red()">Change to Red</button>
<br />
<button type="button" onclick="change_to_blue(true)">Toggle Blue</button>
<button type="button" onclick="change_to_green(true)">Toggle Green</button>
<button type="button" onclick="change_to_red(true)">Toggle Red</button>
<br />
<button type="button" onclick="remove_all()">Remove All Classes</button>
              
            
!

CSS

              
                .blue-text {
  color: blue;
}
.green-text {
  color: green;
}
.red-text {
  color: red;
}

              
            
!

JS

              
                function change_to_blue(toggle = false) {
  const el = document.querySelector("#el");
  if (toggle) {
    el.classList.toggle("blue-text");
  } else {
  el.classList.add("blue-text");
  }
}
function change_to_green(toggle = false) {
  const el = document.querySelector("#el");
  if (toggle) {
    el.classList.toggle("green-text");
  } else {
  el.classList.add("green-text");
  }
}
function change_to_red(toggle = false) {
  const el = document.querySelector("#el");
  if (toggle) {
    el.classList.toggle("red-text");
  } else {
  el.classList.add("red-text");
  }
}
function remove_all() {
  const el = document.querySelector("#el");
  // This clears all the classes
  el.className = "";
}

              
            
!
999px

Console