Package @shinsenter/defer.js

🥇 A super small, super efficient library that helps you lazy load almost everything like images, video, audio, iframes as well as stylesheets, and JavaScript.

Package: @shinsenter/defer.js
Version: 2.4.2
Author: Mai Nhut Tan shin@shin.company
Copyright: 2021 AppSeeds https://code.shin.company/
License: MIT

Use cases

In real life, many resources and third-party scripts, such as jQuery, are used to enhance our website to add additional interactivity, animations, and other effects.

Unfortunately, third-party scripts usually block page rendering and further downloading resources of the page.

There is a common knowledge that you should use <script src="..." async> (or <script src="..." defer>) and/or put your scripts at the very bottom of the page, so that as much as possible of the page gets loaded and rendered to the user, as fast as possible.

But in various cases, using async or defer attributes does not deliver faster page speed than defer.js does. Furthermore defer.js also gives you very simple ways to flexibly optimize other resources in your website.

Why you should consider defer.js

⚡️ Under 1KB (mingzipped size)
🚀 Native API, blazing fast
👍 No dependencies, no jQuery
🧩 Lazy load almost everything
🎯 Core Web Vitals friendly
🔰 Very easy to use
📱 Smartphone browser friendly
✅ Supports legacy browsers (IE9+)
🤝 Works well with your favorite frameworks

Browser support

Works perfectly on modern browsers. Lazy-loading is also available for Internet Explorer 9 ^{* (with IntersectionObserver polyfill library)} and later.

🖥 IE9+ / Microsoft EDGE
🖥 Firefox 4+
🖥 Safari 3+
🖥 Chrome
🖥 Opera
📱 Android 4+
📱 iOS 3.2+

Getting started

Basic

Add defer.min.js from this library into your HTML page, just below the opening <head> tag.

You may download a ZIP of this library, or load it from a CDN like below example.

<head>
  <meta charset="UTF-8" />
  <title>My Awesome Page</title>

  <!-- Put defer.min.js here -->
  <script src="https://cdn.jsdelivr.net/npm/@shinsenter/defer.js@2.4.2/dist/defer.min.js"></script>

  <!-- To support older browsers such as Internet Explorer 9 -->
  <!-- You may want to put IntersectionObserver polyfill right after defer.min.js tag -->
  <script>'IntersectionObserver'in window||document.write('<script src="https://polyfill.io/v3/polyfill.min.js?features=IntersectionObserver"><\/script>');</script>

  <!-- ... -->
</head>

Compatibility with previous releases

I strongly recommend that you should migrate to the latest version for better performance.

If you have no time and want to ensure compatibility with older version, use defer_plus.min.js instead of defer.min.js.

<!-- Put defer_plus.min.js here -->
<script src="https://cdn.jsdelivr.net/npm/@shinsenter/defer.js@2.4.2/dist/defer_plus.min.js"></script>

<!-- To support older browsers such as Internet Explorer 9 -->
<!-- You may want to put IntersectionObserver polyfill right after defer.min.js tag -->
<script>'IntersectionObserver'in window||document.write('<script src="https://polyfill.io/v3/polyfill.min.js?features=IntersectionObserver"><\/script>');</script>

Inlining the library

Since size of defer.js library is optimized to minimum size, you can inline entire library in the <head> of the HTML document to minimize the number of requests.

<head>
  <meta charset="UTF-8" />
  <title>My Awesome Page</title>

  <!-- Inlining defer.min.js -->
  <script>/* content of defer.min.js will be here */</script>

  <!-- ... -->
</head>

Functions

Defer(func, [delay]) ⇒ void
- .all([selector]) ⇒ void
- .js(src, [id], [delay], [callback]) ⇒ void
- .css(src, [id], [delay], [callback]) ⇒ void
- .dom([selector], [delay], [revealedClass], [validator], [observeOptions]) ⇒ void
- .reveal(element, [revealedClass]) ⇒ void
~~defer(func, [delay])~~
~~deferscript(src, [id], [delay], [callback])~~
~~deferstyle(src, [id], [delay], [callback])~~
~~deferimg([selector], [delay], [revealedClass], [validator], [observeOptions])~~
~~deferiframe([selector], [delay], [revealedClass], [validator], [observeOptions])~~

Typedefs

function ⇒ void
closure ⇒ void | bool
Node

Defer(func, [delay]) ⇒ `void`

Used to delay execution of JavaScript which may adversely affect the loading of your web page.

All JavaScript delayed by Defer() will only executed after the web page has completely loaded.

Kind: global function
Since: 2.0

Param	Type	Default	Description
func	`function`		The function that will be deferred.
[delay]	`number`	`0`	The duration in miliseconds to delay the `func` function.

Example
Delay some heavy DOM manipulations in JavaScript.

Defer(function() {
  // Some JavaScript that may block page rendering
  // such as calling jQuery's fadeIn() feature
  jQuery('div').hide().fadeIn().show();
}); // <- script runs after the page has completely loaded

Example
Delay the same JavaScript as above for 3000ms.

Defer(function() {
  jQuery('div').hide().fadeIn().show();
}, 3000); // <- Added 3000 = Delay for 3000ms

Defer(func, [delay]) ⇒ void
- .all([selector]) ⇒ void
- .js(src, [id], [delay], [callback]) ⇒ void
- .css(src, [id], [delay], [callback]) ⇒ void
- .dom([selector], [delay], [revealedClass], [validator], [observeOptions]) ⇒ void
- .reveal(element, [revealedClass]) ⇒ void

Defer.all([selector]) ⇒ `void`

This function is useful for lazy-loading script tags.

All script tags with attribute <script type="deferjs"> will be delayed and automatically executed as soon as the page has completely loaded.

By default, this function is triggered automatically.

Note: For customized deferjs type, please call Defer.all() at the bottom of the <body> tag.

Kind: static method of Defer
Since: 2.0

Param	Type	Default	Description
[selector]	`string`	`"[type=deferjs]"`	A CSS selector that queries script tags will be deferred.

Example
You just need to simply change type="text/javascript" to type="deferjs", or add type="deferjs" to your script tag for it to take effect.

Before:

<script type="text/javascript" src="/path/to/heavy-javascript.js"></script>
<script>
  // Some heavy DOM manipulations here
</script>

After:

<script type="deferjs" src="/path/to/heavy-javascript.js"></script>
<script type="deferjs">
  // Some heavy DOM manipulations here
</script>

Example
If you don't want the <script type="deferjs"> syntax, you can easily choose your own name.

This example uses type="myjs" instead of type="deferjs":

<script type="myjs" src="/path/to/heavy-javascript.js"></script>
<script type="myjs">
  // Some heavy DOM manipulations here
</script>

<!-- Call Defer.all() at the bottom of the `<body>` tag -->
<script>Defer.all('script[type="myjs"]');</script>

Defer.js(src, [id], [delay], [callback]) ⇒ `void`

For lazy loading external JavaScript files.

This function is useful when you don't want heavy JavaScript (especially the widgets of social networks, ad services) to affect your website loading speed.

Kind: static method of Defer
Since: 2.0

Param	Type	Default	Description
src	`string`		URL to the js file that should be lazy loaded.
[id]	`string`		The ID will be assigned to the script tag to avoid downloading the same file multiple times.
[delay]	`number`	`0`	The duration in miliseconds to delay loading the js file.
[callback]	`closure`		The callback function will be executed if the js file is successfully loaded.

Example
Delay loading of Facebook SDK after 3000ms.

Defer.js('https://connect.facebook.net/en_US/sdk.js', 'fb-sdk', 3000);

Example
Delay loading of AddThis SDK after 5000ms.

var addthis_id = 'ra-5c68e61cf456f1cb';
Defer.js('https://s7.addthis.com/js/300/addthis_widget.js#pubid=' + addthis_id, 'addthis-js', 5000);

Defer.css(src, [id], [delay], [callback]) ⇒ `void`

For lazy loading external CSS files.

This function is useful when you don't want heavy CSS (like Web Fonts) to affect your website loading speed.

Kind: static method of Defer
Since: 2.0

Param	Type	Default	Description
src	`string`		URL to the css file that should be lazy loaded.
[id]	`string`		The ID will be assigned to the script tag to avoid downloading the same file multiple times.
[delay]	`number`	`0`	The duration in miliseconds to delay loading the css file.
[callback]	`closure`		The callback function will be executed if the css file is successfully loaded.

Example
Lazy load FontAwesome Webfont from its CDN.

Defer.css('https://pro.fontawesome.com/releases/v5.10.0/css/all.css', 'fa5-css');

Example
Delay loading animate.css from CDN for 1000ms.

Defer.css('https://cdnjs.cloudflare.com/ajax/libs/animate.css/4.1.1/animate.min.css', 'animate-css', 1000);

Defer.dom([selector], [delay], [revealedClass], [validator], [observeOptions]) ⇒ `void`

For lazy loading attributes of any element on the page.

Basically, the Defer.dom function converts all data-* attributes into regular attributes (e.g. from data-src to src) when user scrolling to the position where the element appears within the browser's viewport.

Most of modern browsers support IntersectionObserver feature.

To take advantage of native performance for older browsers that doesn't support this feature (such as IE9), you should load IntersectionObserver polyfill library right after the defer.min.js script tag as following example:

<!-- Put defer.min.js here -->
<script src="https://cdn.jsdelivr.net/npm/@shinsenter/defer.js@2.4.2/dist/defer.min.js"></script>

<!-- Put polyfill right after defer.min.js tag -->
<script>'IntersectionObserver'in window||document.write('<script src="https://polyfill.io/v3/polyfill.min.js?features=IntersectionObserver"><\/script>');</script>

Kind: static method of Defer
Since: 2.0

Param	Type	Default	Description
[selector]	`string`	`"[data-src]"`	A CSS selector that queries elements will be lazy loaded.
[delay]	`number`	`0`	The duration in miliseconds to delay the lazy loading for the elements.
[revealedClass]	`string`		A CSS class will be added automatically after when an element has been successfully revealed.
[validator]	`closure`		A function will be executed with element will be lazy loaded as its argument. If the function returns `false`, lazy loading for that element will be skipped.
[observeOptions]	`object`		Intersection observer options

Example
Basic usage: Lazy load all <img> tags which have CSS class lazy.

<script>Defer.dom('img.lazy');</script>

<!-- Here may be a very long content -->

<img class="lazy" alt="Photo 1" data-src="https://picsum.photos/200/300?random=1" width="200" height="300" />
<img class="lazy" alt="Photo 2" data-src="https://picsum.photos/200/300?random=2" width="200" height="300" />
<img class="lazy" alt="Photo 3" data-src="https://picsum.photos/200/300?random=3" width="200" height="300" />

Example
Basic usage: Lazy load background image of a div tag.

<style>
  #my_div {
  width: 300px;
  height: 200px;
  }
</style>

<script>
  // Lazy load div tag which has id="my_div"
  Defer.dom('#my_div');
</script>

<!-- Here may be a very long content -->

<div id="my_div"
  data-style="background: url(https://img.youtube.com/vi/Uz970DggW7E/hqdefault.jpg) 50% 50% / cover no-repeat;">
  <!-- The content -->
</div>

Example
Advanced usage: Delay lazy loading <img> tags 200ms after the page has completely loaded. Then it will add a CSS class loaded to the fully lazy loaded image element.

<script>Defer.dom('img.lazy', 200, 'loaded');</script>

<!-- Here may be a very long content -->

<img class="lazy" alt="Photo 1" data-src="https://picsum.photos/200/300?random=4" width="200" height="300" />
<img class="lazy" alt="Photo 2" data-src="https://picsum.photos/200/300?random=5" width="200" height="300" />
<img class="lazy" alt="Photo 3" data-src="https://picsum.photos/200/300?random=6" width="200" height="300" />

Example
Advanced usage: Lazy load with Intersection observer options

<script>
  // Preload images within 200% of the current viewport size.
  Defer.dom("img.early-lazy", 200, "loaded", null, {
  rootMargin: "200%"
  });
</script>

<!-- Here may be a very long content -->

<img class="early-lazy" alt="Photo 1" data-src="https://picsum.photos/200/300?random=7" width="200" height="300" />
<img class="early-lazy" alt="Photo 2" data-src="https://picsum.photos/200/300?random=8" width="200" height="300" />
<img class="early-lazy" alt="Photo 3" data-src="https://picsum.photos/200/300?random=9" width="200" height="300" />

Example
We can use CSS class that added to the lazy loaded element to add animation to the successfully loaded elements.

<script>Defer.dom('img.fade', 200, 'loaded');</script>
<style>
  img.fade {
    transition: opacity 500ms ease-in-out;
    opacity: 0;
  }
  img.fade.loaded {
    background: none;
    opacity: 1;
  }
</style>

<!-- Here may be a very long content -->

<img class="fade" alt="Photo 1" data-src="https://picsum.photos/200/300?random=10" width="200" height="300" />
<img class="fade" alt="Photo 2" data-src="https://picsum.photos/200/300?random=11" width="200" height="300" />
<img class="fade" alt="Photo 3" data-src="https://picsum.photos/200/300?random=12" width="200" height="300" />

Example
This function can be used similarly for other tags such as <iframe>, <video>, <audio>, <picture> tags.

<script>
  // Lazy load all elements which have CSS class `multi-lazy`
  Defer.dom('.multi-lazy', 200, 'loaded');
</script>

<!-- Here may be a very long content -->

<iframe class="multi-lazy" title="Youtube"
  width="400" height="300" allowfullscreen
  allow="accelerometer;autoplay;encrypted-media;gyroscope;picture-in-picture"
  data-style="background: url(https://img.youtube.com/vi/Uz970DggW7E/hqdefault.jpg) 50% 50% / cover no-repeat;"
  data-src="https://www.youtube.com/embed/Uz970DggW7E"></iframe>

<picture class="multi-lazy">
  <source media="(min-width:800px)" data-srcset="https://picsum.photos/800/1200">
  <source media="(min-width:600px)" data-srcset="https://picsum.photos/600/900">
  <img data-src="https://picsum.photos/200/300" alt="Photo" style="width:auto;">
</picture>

<audio class="multi-lazy" controls>
  <source data-src="sound.ogg" type="audio/ogg">
  <source data-src="sound.mp3" type="audio/mpeg">
  Your browser does not support the audio tag.
</audio>

<video class="multi-lazy" width="320" height="240" controls>
  <source data-src="movie.mp4" type="video/mp4">
  <source data-src="movie.ogg" type="video/ogg">
  Your browser does not support the video tag.
</video>

Example
Or even execute a piece of JavaScript when the user scrolls to the element #scroll_reveal.

<script>
  // Show an alert when user scrolled to #scroll_reveal
  Defer.dom('#scroll_reveal', null, null, function(element) {
    window.alert('You scrolled to #' + element.id);
  });
</script>

<!-- Here may be a very long content -->

<div id="scroll_reveal">
  This is my content.
</div>

Example
Combine with other Defer functions. Delay loading highlightjs library for 1000ms. Then when you scroll to any code tag, enable code highlighting for it.

var base = 'https://cdn.jsdelivr.net/npm/highlightjs@9.12.0';
Defer.css(base + '/styles/rainbow.css', 'hljs-css', 1000);
Defer.js(base + '/highlight.pack.min.js', 'hljs-js', 1000, function () {
  Defer.dom('pre code', 0, 'ide-loaded', function (block) {
    hljs.highlightBlock(block);
  });
});

Defer.reveal(element, [revealedClass]) ⇒ `void`

Reveal an element which is lazyloaded by the library

Kind: static method of Defer
Since: 2.1

Param	Type	Description
element	`Node`	The DOM Node element
[revealedClass]	`string`	A CSS class will be added automatically after when an element has been successfully revealed.

Example

// Show single element
var node = document.getElementById('my-video');
Defer.reveal(node);

// Show multiple elements
document.querySelectorAll('.multi-lazy')
  .forEach(function(node) {
    Defer.reveal(node);
  });

// Or even shorter way
document.querySelectorAll('.multi-lazy').forEach(Defer.reveal);

// Add 'loaded' class name after revealed elements
document.querySelectorAll('.multi-lazy')
  .forEach(function(node) {
    Defer.reveal(node, 'loaded');
  });

defer(func, [delay])

Deprecated

Deprecated since version 2.0

Kind: global function
See: Defer
Since: 1.0

Param	Type
func	`function`
[delay]	`number`

deferscript(src, [id], [delay], [callback])

Deprecated

Deprecated since version 2.0

Kind: global function
See: js
Since: 1.0

Param	Type
src	`string`
[id]	`string`
[delay]	`number`
[callback]	`callback`

deferstyle(src, [id], [delay], [callback])

Deprecated

Deprecated since version 2.0

Kind: global function
See: css
Since: 1.0

Param	Type
src	`string`
[id]	`string`
[delay]	`number`
[callback]	`callback`

deferimg([selector], [delay], [revealedClass], [validator], [observeOptions])

Deprecated

Deprecated since version 2.0

Kind: global function
See: dom
Since: 1.0

Param	Type
[selector]	`string`
[delay]	`number`
[revealedClass]	`string`
[validator]	`callback`
[observeOptions]	`object`

deferiframe([selector], [delay], [revealedClass], [validator], [observeOptions])

Deprecated

Deprecated since version 2.0

Kind: global function
See: dom
Since: 1.0

Param	Type
[selector]	`string`
[delay]	`number`
[revealedClass]	`string`
[validator]	`callback`
[observeOptions]	`object`

function ⇒ `void`

A definition for an ordinary function, used as a parameter to another function.

Kind: global typedef

closure ⇒ `void` | `bool`

The definition for a function that takes one parameter is a DOM Node element

Kind: global typedef

Param	Type	Description
element	`Node`	The DOM Node element

Node

The DOM Node interface

Kind: global typedef
See: https://developer.mozilla.org/en-US/docs/Web/API/Node

Defer.js for another platforms

PHP library

https://github.com/shinsenter/defer.php/

🚀 A PHP library that focuses on minimizing payload size of HTML document and optimizing processing on the browser when rendering the web page.

Wordpress plugin

https://github.com/shinsenter/defer-wordpress/

⚡️ A native, blazing fast lazy loader. ✅ Legacy browsers support (IE9+). 💯 SEO friendly. 🧩 Lazy load almost anything.

Laravel package

https://github.com/shinsenter/defer-laravel/

🚀 A Laravel package that focuses on minimizing payload size of HTML document and optimizing processing on the browser when rendering the web page.

Support my activities

From Vietnam 🇻🇳 with love.

Name		Name	Last commit message	Last commit date
Latest commit History 156 Commits
.build		.build
.github		.github
dist		dist
docs		docs
src		src
.editorconfig		.editorconfig
.gitattributes		.gitattributes
.gitignore		.gitignore
CONTRIBUTING.md		CONTRIBUTING.md
LICENSE		LICENSE
README.md		README.md
package-lock.json		package-lock.json
package.json		package.json

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Repository files navigation

Package @shinsenter/defer.js

Use cases

Why you should consider defer.js

Browser support

Getting started

Basic

Compatibility with previous releases

Inlining the library

Functions

Typedefs

Defer(func, [delay]) ⇒ `void`

Defer.all([selector]) ⇒ `void`

Defer.js(src, [id], [delay], [callback]) ⇒ `void`

Defer.css(src, [id], [delay], [callback]) ⇒ `void`

Defer.dom([selector], [delay], [revealedClass], [validator], [observeOptions]) ⇒ `void`

Defer.reveal(element, [revealedClass]) ⇒ `void`

defer(func, [delay])

deferscript(src, [id], [delay], [callback])

deferstyle(src, [id], [delay], [callback])

deferimg([selector], [delay], [revealedClass], [validator], [observeOptions])

deferiframe([selector], [delay], [revealedClass], [validator], [observeOptions])

function ⇒ `void`

closure ⇒ `void` | `bool`

Node

Defer.js for another platforms

PHP library

Wordpress plugin

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Package @shinsenter/defer.js

Use cases

Why you should consider defer.js

Browser support

Getting started

Basic

Compatibility with previous releases

Inlining the library

Functions

Typedefs

Defer(func, [delay]) ⇒ void

Defer.all([selector]) ⇒ void

Defer.js(src, [id], [delay], [callback]) ⇒ void

Defer.css(src, [id], [delay], [callback]) ⇒ void

Defer.dom([selector], [delay], [revealedClass], [validator], [observeOptions]) ⇒ void

Defer.reveal(element, [revealedClass]) ⇒ void

defer(func, [delay])

deferscript(src, [id], [delay], [callback])

deferstyle(src, [id], [delay], [callback])

deferimg([selector], [delay], [revealedClass], [validator], [observeOptions])

deferiframe([selector], [delay], [revealedClass], [validator], [observeOptions])

function ⇒ void

closure ⇒ void | bool

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Defer.js for another platforms

PHP library

Wordpress plugin

Laravel package

Support my activities

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Defer(func, [delay]) ⇒ `void`

Defer.all([selector]) ⇒ `void`

Defer.js(src, [id], [delay], [callback]) ⇒ `void`

Defer.css(src, [id], [delay], [callback]) ⇒ `void`

Defer.dom([selector], [delay], [revealedClass], [validator], [observeOptions]) ⇒ `void`

Defer.reveal(element, [revealedClass]) ⇒ `void`

function ⇒ `void`

closure ⇒ `void` | `bool`

Packages