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DOM Manipulation

The Document Object Model (DOM) is the browser’s representation of your HTML. JavaScript provides powerful APIs to manipulate the DOM and create dynamic, interactive web pages.

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Show and Hide Elements

JavaScript allows you to change the CSS properties of an element by accessing its style property. This way, you can show or hide HTML elements by changing their display property.

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Hide Elements

In order to hide an HTML element, you can use the display: none CSS property. This will remove the element from the page layout, but it will still be present in the DOM. 
const hide = (...el) => [...el].forEach(e => (e.style.display = 'none'));

hide(...document.querySelectorAll('img'));
// Hides all <img> elements on the page

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Show Elements

Most HTML elements have a default display property value. For example, the default value for <div> elements is block, while the default value for <span> elements is inline. In order to show an element, you can set its display property to its default value, or to an empty string (''). 
const show = (...el) => [...el].forEach(e => (e.style.display = ''));

show(...document.querySelectorAll('img'));
// Shows all <img> elements on the page
Combining the spread operator (...) with Array.prototype.forEach() allows you to show or hide multiple elements at once.

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Rendering DOM Elements

Have you ever wondered how React’s rendering works under the hood? Here’s a simple JavaScript function that renders a DOM tree in a specified container.

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Representing a DOM Tree

Each element is an object with a type property representing the element’s tag name, and a props object containing the element’s attributes, event listeners, and children. 
const myElement = {
  type: 'button',
  props: {
    type: 'button',
    className: 'btn',
    onClick: () => alert('Clicked'),
    children: [{ props: { nodeValue: 'Click me' } }]
  }
};
// Equivalent to:
//  <button
//    type="button"
//    class="btn"
//    onClick="alert('Clicked')"
//  >
//    Click me
//  </button>
The special case of text elements is represented by an object without a type property, only containing a props object with a nodeValue property.

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Render Function

const renderElement = ({ type, props = {} }, container) => {
  const isTextElement = !type;
  const element = isTextElement
    ? document.createTextNode('')
    : document.createElement(type);

  const isListener = p => p.startsWith('on');
  const isAttribute = p => !isListener(p) && p !== 'children';

  Object.keys(props).forEach(p => {
    if (isAttribute(p)) element[p] = props[p];
    if (!isTextElement && isListener(p))
      element.addEventListener(p.toLowerCase().slice(2), props[p]);
  });

  if (!isTextElement && props.children && props.children.length)
    props.children.forEach(childElement =>
      renderElement(childElement, element)
    );

  container.appendChild(element);
};

const myElement = {
  type: 'button',
  props: {
    type: 'button',
    className: 'btn',
    onClick: () => alert('Clicked'),
    children: [{ props: { nodeValue: 'Click me' } }]
  }
};

renderElement(myElement, document.body);
// Renders our <button> element in the body of the document
This implementation is for demonstration purposes only and lacks many features and optimizations present in React or Preact.

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Creating Elements

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createElement

const div = document.createElement('div');
div.className = 'container';
div.textContent = 'Hello, World!';
document.body.appendChild(div);

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createTextNode

const text = document.createTextNode('Hello, World!');
document.body.appendChild(text);

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Modifying Elements

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Text Content

const element = document.getElementById('my-element');
element.textContent = 'New text';
element.innerHTML = '<strong>New HTML</strong>';
Use textContent for plain text and innerHTML for HTML. Be careful with innerHTML when dealing with user input to prevent XSS attacks.

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Attributes

const element = document.getElementById('my-element');

// Get attribute
const value = element.getAttribute('data-id');

// Set attribute
element.setAttribute('data-id', '123');

// Remove attribute
element.removeAttribute('data-id');

// Check if attribute exists
if (element.hasAttribute('data-id')) {
  console.log('Has data-id attribute');
}

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Classes

const element = document.getElementById('my-element');

// Add class
element.classList.add('active');

// Remove class
element.classList.remove('inactive');

// Toggle class
element.classList.toggle('highlighted');

// Check if has class
if (element.classList.contains('active')) {
  console.log('Element is active');
}

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Styles

const element = document.getElementById('my-element');

// Set individual style
element.style.color = 'red';
element.style.fontSize = '16px';

// Set multiple styles
Object.assign(element.style, {
  color: 'blue',
  fontSize: '18px',
  padding: '10px'
});

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Removing Elements

// Remove element
const element = document.getElementById('my-element');
element.remove();

// Remove child
const parent = document.getElementById('parent');
const child = document.getElementById('child');
parent.removeChild(child);

// Remove all children
const container = document.getElementById('container');
while (container.firstChild) {
  container.removeChild(container.firstChild);
}

// Or more simply
container.innerHTML = '';

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Inserting Elements

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appendChild

const parent = document.getElementById('parent');
const child = document.createElement('div');
parent.appendChild(child);

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insertBefore

const parent = document.getElementById('parent');
const newElement = document.createElement('div');
const referenceElement = document.getElementById('reference');
parent.insertBefore(newElement, referenceElement);

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insertAdjacentHTML

const element = document.getElementById('my-element');

// Before the element itself
element.insertAdjacentHTML('beforebegin', '<p>Before</p>');

// Just inside the element, before its first child
element.insertAdjacentHTML('afterbegin', '<p>First child</p>');

// Just inside the element, after its last child
element.insertAdjacentHTML('beforeend', '<p>Last child</p>');

// After the element itself
element.insertAdjacentHTML('afterend', '<p>After</p>');

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Cloning Elements

const original = document.getElementById('original');

// Shallow clone (no children)
const shallowClone = original.cloneNode(false);

// Deep clone (with children)
const deepClone = original.cloneNode(true);

document.body.appendChild(deepClone);

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Best Practices

DOM manipulation is expensive. Batch changes together and use DocumentFragment for multiple insertions.
classList provides methods like add, remove, toggle, and contains, making it easier and more reliable than manipulating className string.
When using innerHTML with user input, always sanitize the content to prevent XSS attacks. Consider using textContent instead.
If you need to access an element multiple times, cache the reference instead of querying the DOM repeatedly.

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