Element querySelector(
String selectors
)

Finds the first descendant element of this document that matches the specified group of selectors.

Unless your webpage contains multiple documents, the top-level querySelector method behaves the same as this method, so you should use it instead to save typing a few characters.

selectors should be a string using CSS selector syntax.

var element1 = document.querySelector('.className');
var element2 = document.querySelector('#id');

For details about CSS selector syntax, see the CSS selector specification.

Source

/**
 * Finds the first descendant element of this document that matches the
 * specified group of selectors.
 *
 * Unless your webpage contains multiple documents, the top-level
 * [querySelector]
 * method behaves the same as this method, so you should use it instead to
 * save typing a few characters.
 *
 * [selectors] should be a string using CSS selector syntax.
 *
 *     var element1 = document.querySelector('.className');
 *     var element2 = document.querySelector('#id');
 *
 * For details about CSS selector syntax, see the
 * [CSS selector specification](http://www.w3.org/TR/css3-selectors/).
 */
Element querySelector(String selectors) => document.querySelector(selectors);