Please use template literal for long string contain many variable.
More info https://developer.mozilla.org/id/docs/Web/JavaScript/Reference/Template_literals
// === BAD
const stringComplex1 = '<div class="' + varA + '"><span>text ' + varB + '</span></div>';
// === GOOD
const stringComplex2 = `<div class="${varA}">
<span>text ${varB}</span>
</div>`;