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Showing content from https://developer.mozilla.org/en-US/docs/Web/API/Element/querySelector below:

Element: querySelector() method - Web APIs

Baseline Widely available

The querySelector() method of the Element interface returns the first element that is a descendant of the element on which it is invoked that matches the specified group of selectors.

selectors

A string containing one or more selectors to match. This string must be a valid CSS selector string; if it isn't, a SyntaxError exception is thrown.

Note that the HTML specification does not require attribute values to be valid CSS identifiers. If a class or id attribute value is not a valid CSS identifier, then you must escape it before using it in a selector, either by calling CSS.escape() on the value, or using one of the techniques described in Escaping characters. See Escaping attribute values for an example.

The first descendant element of baseElement which matches the specified group of selectors. The entire hierarchy of elements is considered when matching, including those outside the set of elements including baseElement and its descendants; in other words, selectors is first applied to the whole document, not the baseElement, to generate an initial list of potential elements. The resulting elements are then examined to see if they are descendants of baseElement. The first match of those remaining elements is returned by the querySelector() method.

If no matches are found, the returned value is null.

SyntaxError DOMException

Thrown if the specified selectors are invalid.

Let's consider a few examples.

In this first example, the first <style> element which either has no type or has type "text/css" in the HTML document body is returned:

const el = document.body.querySelector(
  "style[type='text/css'], style:not([type])",
);

This example uses the :scope pseudo-class to retrieve direct children of the parentElement element.

<div>
  <h6>Page Title</h6>
  <div id="parent">
    <span>Love is Kind.</span>
    <span>
      <span>Love is Patient.</span>
    </span>
    <span>
      <span>Love is Selfless.</span>
    </span>
  </div>
</div>

span {
  display: block;
  margin-bottom: 5px;
}
.red span {
  background-color: red;
  padding: 5px;
}

const parentElement = document.querySelector("#parent");
let allChildren = parentElement.querySelectorAll(":scope > span");
allChildren.forEach((item) => item.classList.add("red"));

This example demonstrates that the hierarchy of the entire document is considered when applying selectors, so that levels outside the specified baseElement are still considered when locating matches.

<div>
  <h5>Original content</h5>
  <p>
    inside paragraph
    <span>inside span</span>
    inside paragraph
  </p>
</div>
<div>
  <h5>Output</h5>
  <div id="output"></div>
</div>

const baseElement = document.querySelector("p");
document.getElementById("output").textContent =
  baseElement.querySelector("div span").textContent;

The result looks like this:

Notice how the "div span" selector still successfully matches the <span> element, even though the baseElement's child nodes do not include the <div> element (it is still part of the specified selector).

This example shows that if an HTML document contains an id which is not a valid CSS identifier, then we must escape the attribute value before using it in querySelector().

In the following code, a <div> element has an id of "this?element", which is not a valid CSS identifier, because the "?" character is not allowed in CSS identifiers.

We also have three buttons, and a <pre> element for logging errors.

<div id="container">
  <div id="this?element"></div>
</div>

<button id="no-escape">No escape</button>
<button id="css-escape">CSS.escape()</button>
<button id="manual-escape">Manual escape</button>

<pre id="log"></pre>

div {
  background-color: blue;
  margin: 1rem 0;
  height: 100px;
  width: 200px;
}

All three buttons, when clicked, try to select the <div>, and then set its background color to a random value.

• The first button uses the "this?element" value directly.
• The second button escapes the value using CSS.escape().
• The third button explicitly escapes the "?" character using a backslash. Note that we must also escape the backslash itself, using another backslash, like: "\\?".

const container = document.querySelector("#container");
const log = document.querySelector("#log");

function random(number) {
  return Math.floor(Math.random() * number);
}

function setBackgroundColor(id) {
  log.textContent = "";

  try {
    const element = container.querySelector(`#${id}`);
    const randomColor = `rgb(${random(255)} ${random(255)} ${random(255)})`;
    element.style.backgroundColor = randomColor;
  } catch (e) {
    log.textContent = e;
  }
}

document.querySelector("#no-escape").addEventListener("click", () => {
  setBackgroundColor("this?element");
});

document.querySelector("#css-escape").addEventListener("click", () => {
  setBackgroundColor(CSS.escape("this?element"));
});

document.querySelector("#manual-escape").addEventListener("click", () => {
  setBackgroundColor("this\\?element");
});

Clicking the first button gives an error, while the second and third buttons work properly.

See Document.querySelector() for additional examples of the proper format for the selectors.

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