Dynamic Page Loader

An abstract / multi-purpose component that loads parts of a page dynamically. Intends to solve:

• paginations
• infinite scroll (TBD, needs append mode)
• facets

Loads content of a web page dynamically when the user navigates to a different URL. It thereby enables seamless page transitions between two different static web pages, similar to the ones provided by Next.js or Gatsby.

Supports:

• Elements that are preserved in the DOM when the page changes (data-preserve-id)
• Hook to test if the given URL should be loaded dynamically
• Hook to execute a script before the page content is changed (e.g. for animations)
• Hook to execute a script after the page content was changed (e.g. for animations)
• Only adds dynamic click handler to elements once (to not fire url change multiple times for elements that are preserved over a page change)

Attention!

• For now only modifies HTMLElements – and no other node types (text, comments, etc.)

Important

There are certain limitations concerning the use of JavaScript, as the global scope is preserved over page loads:

• Don't use any global variables or functions, especially not consts (would throw an already declared error). Use IFFEs e.g. where appropriate.
• Don't use type="module" script tags for code that should be executed on page load. Module script tags will only execute when the page is initially loaded.

Be aware that CSS is handled differently:

• The sort order of <link> elements will not be adjusted to the element order of the new page, as moving <link> elements around causes flickering.

Example

import {
    applyChanges,
    handleLinkClicks,
    handlePopState,
    loadFile,
    createNode,
    canBeIdentical,
    isIdentical,
    applyAttributes,
} from '@joinbox/ui-components/DynamicPageLoader';

// Prevents loading of a new page when a link is clicked. Uses pushState to update the URL 
// displayed by the browser instead and emits 'urlchange' event that will be handled later.
handleLinkClicks({
    // In this case, we hijack all clicks on regular links
    linkElements: document.querySelectorAll('a'),
    // Limit the links that are hijacked; in this case, only use dynamic page loader for absolute
    // links (on current domain)
    checkLink: link => link.startsWith('/'),
});

// Handles navigation through the back button and emits 'urlchange' event if the previous page
// was loaded dynamically.
handlePopState();


// Add event listener to 'urlchange' event that was fired above. Handle URL change with a smooth
// transition instead of a hard page load.
window.addEventListener(
    'urlchange',
    async(ev) => {
        const { url } = ev.detail;
        const dom = await loadFile(url);

        // If you like, add some nice animations here.

        // <script> tags must be created through document.createElement and appended to DOM in order
        // to be executed; we generally use innerHTML to change DOM, which does not execute script
        // elements.
        const updateNode = node => (node.tagName === 'SCRIPT' ? createNode(document, node) : node);

        // Update body
        applyChanges({
            originalNode: document.querySelector('body'),
            newNode: dom.querySelector('body'),
            canBeIdentical,
            isIdentical,
            updateNode,
            updateAttributes: applyAttributes,
        });

        // Update head
        applyChanges({
            originalNode: document.querySelector('head'),
            newNode: dom.querySelector('head'),
            canBeIdentical,
            isIdentical,
            updateNode,
            updateAttributes: applyAttributes,
        });

        // To update a preserved DOM element, use a minimal timeOut; if we add the class while
        // the DOM element is being moved, transitions will not happen.
        setTimeout(() => {
            const method = url.includes('about') ? 'add' : 'remove';
            document.querySelector('.header').classList[method]('about');
        });

    },

    // Note once here which is crucial; window will persist over all page changes. If we add
    // the urlchange handler every time, it will fire many times after many page reloads.
    { once: true },
);

Functions

handleLinkClicks

Adds click event listener to linkElements passed in; executes pushState on click and dispatches urlchange event on window.

Parameters

• linkElements: iterable collection of DOM elements (i.e. links) whose behaviour should be 'hijacked' and not trigger a page refresh in the browser.
• checkLink: function that takes a single parameter url that corresponds to the link's `href`` attribute. Return falsy value if the link should be handled by the browser and not by DynamicPageLoader.

handlePopState

Dispatches a urlchange event on window when a popState event occurs on window.

Parameters

None

loadFile

Loads a remote file (through fetch) and returns its content as a DOM tree.

Parameters

• url: URL to load

createNode

Takes a HTMLElement and creates a new element (through document.createElement) with the same tag name and attributes. Needed to e.g. create a proper script tag.

Parameters

• document: reference to the document that the element should be created for
• node: HTMLElement that should be cloned/created

applyChanges

Takes two DOM nodes and applies the changes from newNode to originalNode.

Parameters

• originalNode: HTMLElement that will be patched with the changes in newNode
• newNode: HTMLElement whose contents will be added to/removed from originalNode
• canBeIdentical: function that thakes a single parameter element (HTMLElement); return true if the element might be preserved (isIdentical might return true). Only needed to improve speed.
• isIdentical: function that takes two parameters (both HTMLElements) and returns true if both elements are considered identical. The original element will be preserved in the DOM, the new element will be ignored.
• updateNode (optional): function that takes a single parameter (HTMLElement) and is expected to return a modified HTMLElement, if you wish to change the HTMLElement before it is added to the DOM. Needed to e.g. create a <script> element from scratch to make sure it is executed.
• updateAttributes (optional): function that takes two parameters (newElement, originalelement, both HTMLElements) and copies attributes from preserved newElement to originalElement. By default, applyChanges does not update attributes on preserved elements.

canBeIdentical

A default function for the canBeIdentical argument of applyChanges. Returns true if

• both elements are link or meta elements
• element has an attribute data-preserve-id.

isIdentical

A default function for the isIdentical argument of applyChanges. Returns true if

• both elements are link or meta elements and have exactly the same attributes
• both elements have a data-preserve-id attribute that is identical.

applyAttributes

A default function for the updateAttributes argument of applyChanges. Copies all changed or new attributes from origin to target and removes the ones from target that are not present on origin.