src/utils/src-loader.js
function validateAndGetQuerySelector (selector) {
  try {
    var el = document.querySelector(selector);
    if (!el) {
      warn('No element was found matching the selector: "%s"', selector);
    }
    return el;
  } catch (e) {  // Capture exception if it's not a valid selector.
    warn('"%s" is not a valid selector', selector);
    return undefined;
  }
}
function validateAndGetQuerySelector (selector) {
  try {
    var el = document.querySelector(selector);
    if (!el) {
      warn('No element was found matching the selector: "%s"', selector);
    }
    return el;
  } catch (e) {  // Capture exception if it's not a valid selector.
    warn('"%s" is not a valid selector', selector);
    return undefined;
  }
}
function validateAndGetQuerySelector ( selector ) { try { var el = document . querySelector ( selector ) ; if ( ! el ) { warn ( 'No element was found matching the selector: "%s"' , selector ) ; } return el ; } catch ( e ) { // Capture exception if it's not a valid selector. warn ( '"%s" is not a valid selector' , selector ) ; return undefined ; } }
Query and validate a query selector,

@param  {string} selector - DOM selector.
@return {object|null|undefined} Selected DOM element if exists.
null if query yields no results.
undefined if `selector` is not a valid selector.
Query and validate a query selector