convert html to html string in JS file

update your JS file to:

function createHtmlSection() {
    return `
             <li class="chapter up-wrapper-btn">
                <div>
                  <button>
                     <i class="fa fa-plus" onclick="addSection('up',this)"></i> 
                  </button>
                  <label contenteditable="true">section 1</label>
               </div>
             </li>
          `
}

Read this link for more information: template literals

You can use a template literal (note the back-ticks). The literal supports multiline, and you won't need to escape the quotes (you'll need to escape back-ticks).

`<li class="chapter up-wrapper-btn">
    <div>
       <button><i class="fa fa-plus" onclick="addSection('up',this)"></i></button>
        <label contenteditable="true">section 1</label>
     </div>
</li>`

Example:

function createHtmlSection() {
  return `
    <li class="chapter up-wrapper-btn">
        <div>
           <button><i class="fa fa-plus" onclick="addSection('up',this)"></i></button>
            <label contenteditable="true">section 1</label>
         </div>
    </li>
  `;
}


document.querySelector('#root')
  .innerHTML = createHtmlSection();

<ul id="root"></ul>

You can also pass parameters to the function, and insert them to the string using expression interpolation:

function createHtmlSection(label) {
  return `
    <li class="chapter up-wrapper-btn">
        <div>
           <button><i class="fa fa-plus" onclick="addSection('up',this)"></i></button>
            <label contenteditable="true">${label}</label>
         </div>
    </li>
  `;
}


document.querySelector('#root')
  .innerHTML = createHtmlSection('!!! section !!!');

<ul id="root"></ul>