lazyload-css 😴 ·

Promise based method for adding a style sheet to the page if it has not already been added

Install

yarn add lazyload-css

bower install lazyload-css --save

Weigh In

Imported Weight

When used with require() you'll notice very little weight is added to your bundle.

const lazyLoadCSS = require('lazyLoadCSS');

VanillaJS Weight

The UMD module wrapper weighs more than the lazyLoadCSS() method itself.
If you want to go rogue, you can load directly from source.

Usage

lazyLoadCSS accepts two parameters. The path to the script to load and an optional id or configuration Object.

lazyLoadCSS('css/main.css', 'main').then(() => {
  // main.css is loaded now with an id of main
})

The id parameter is optional. It is used to ensure that subsequent requests to load a script with that same id immediately resolve. If you omit the id parameter, the DOM will first be queried for a <link> with the same href attribute, before making a new request by appending a new <link> tag.

lazyLoadCSS uses this id to ensure scripts with the same id are only loaded once. This allows web components to request dependencies with lazyLoadCSS and rest assured the sheets will only be loaded once regardless of how many times they are requested.

lazyLoadCSS is packaged as a UMD module so it can be included in several ways.

The UMD pattern typically attempts to offer compatibility with the most popular script loaders of the day (e.g RequireJS amongst others). In many cases it uses AMD as a base, with special-casing added to handle CommonJS compatibility.
 — umd

With require()

const lazyLoadCSS = require(`lazyLoadCSS`);
lazyLoadCSS('main.css', 'main').then(() => {
  /// main.css loaded
});
 

With VanillaJS

lazyLoadCSS('main.css', 'main').then(() => {
  /// main.css loaded
});

Multiple stylesheets can be asynchronously loaded by passing an Array of lazyLoadCSS promises to Promise.all().

  Promise.all([
    lazyLoadCSS("assets/css/base.css", "base"),
    lazyLoadCSS("assets/css/layout.css", "layout")
  ]).then(() => {
    // stylesheets are loaded now
  });

Configuration

lazyLoadCSS accepts two parameters. The path to the script to load and an optional id or configuration Object.

In the below example, unless lazyLoadCSS already loaded a <link> with the same id or href, <link type="text/css" rel="stylesheet" media="all" href="main.css" /> will be appended to document.head.

lazyLoadCSS('main.css', {
  id: 'main',
  media: 'screen'
}).then((link) => {
  // link is either the newly added stylesheet or the one that was already there
});

In the next example, a stylesheet will forcefully be added regardless of if one with the same href or id already exists.

lazyLoadCSS('print.css', {
  id: 'print',
  media: 'print',
  force: true
}).then((link) => {
  // link is the newly added stylesheet
});

See Also

• lazyload-script

✅ Getting Started

We're going to use yarn so make sure that is installed.

npm install yarn -g

Now clone the repo and run the tests.

git clone -b master git://github.com/jpdevries/lazyload-css.git
cd lazyload-css
yarn
yarn test