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srcset-loader

srcset-loader for webpack 2

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A super flexible and easily chainable (with other loaders like file-loader or image-webpack-loader) srcset loader for webpack 2.

Its purpose is to automatically resize your images to the requested dimensions and return those as an srcSet.

Installation

npm i -D srcset-loader

Usage

There are basically two parts to configure, one part is the loader itself, the other part is the images you want to have a srcset for. Instead of specifying the loader inline, you specify which images should be loaded with the srcset-loader in your webpack config, then specify the sizes for the srcset on the resource itself.

e.g. your config could have a loader specified like this, notice the ?sizes at the end of the resource match

const webpackConfig = {
  // Only showing relevant parts.
  module: {
    rules: [{
      // match image files
      test: /\.(jpe?g|png|svg|gif)$/,
 
      // match one of the loader's main parameters (sizes and placeholder)
      resourceQuery: /[?&](sizes|placeholder)(=|&|\[|$)/,
 
      use: [
        'srcset-loader',
 
        // any other loader
        'file-loader?hash=sha512&digest=hex&name=[hash].[ext]',
        'image-webpack-loader?optimizationLevel=7&interlaced=false',
      ],
    }],
  },
 
  // ...
};

and a resource request could look like this:

import image from './image.jpeg?sizes=200w+800w';

This allows us to separate the configuration of the loader, and the specification of the image sizes of the srcset. This also allows us to use the same loaders we use for normal image requests (e.g. optimizers like webpack-image-loader or the file-loader). Basically all this loader does is creating multiple requests for each srcset image and let known and well tested loaders do the heavy lifting.

Why is the srcset loader before the other loaders

Its actually very important that you add the srcset-loader before all the other loaders, as the srcset-loader uses webpack's pitch mechanism to split your import request into multiple import request which will all use the loaders specified after srcset-loader.

Simplified it works something like this:

// webpack gets this request
const someSrcSet = require('./someImage.jpg?sizes=800w+500w+200w');
 
//----------------------------------------
 
// the srcset loader gets this *single* request that actually looks something like this:
const someSrcSet = require('srcset-loader!url-loader!image-webpack-loader!./someImage.jpg?sizes=800w+500w+200w');
 
// and splits it up into *three* requests looking more like this:
const someSrcSet = [
  require('url-loader!image-webpack-loader!srcset-loader/resize!./someImage.jpg?size=800'),
  require('url-loader!image-webpack-loader!srcset-loader/resize!./someImage.jpg?size=500'),
  require('url-loader!image-webpack-loader!srcset-loader/resize!./someImage.jpg?size=200'),
];

Thats all :), simple and robust.

Can I use srcset-loader inline?

Yes you can, but why? Seriously why? Please write an issue.

srcset-loader options

sizes

sizes is the main feature of the srcset-loader, you use this option to specify the different image sizes you wish to import.

You can either specify the different sizes as a standard array (?sizes[]=100w&sizes[]=200w) or using the less verbose, srcset-specific, syntax (?sizes=100w+200w)

The sizes values must follow the format <number>w (where w stands for width), or the string default (not resized). <density>x is not a supported format.

lightweight

This will remove any property that can be computed at runtime in order to reduce the size of the bundle.

placeholder

placeholder will generate and inline a tiny placeholder which you can display while the full-sized image loads.

You can specify the size of the placeholder as the value of the option (e.g. placeholder=12). By default, the size is 20px (width).

Results of an image loaded with srcset-loader

An image imported with the srcset-loader returns an Object which contains the following properties:

srcSet: string

This property contains the string to use as the value of HTMLImageElement's srcset (i.e. <img srcset="..."/>).

e.g.

import image from './image.jpeg?sizes=200w+800w';
// image.srcSet => 'xxxx.jpeg 200w,xxxx.jpeg 800w'

Note: This key is not available if lightweight is specified in either the Loader options or the resource query.

sources: object

The sources object is a map where the keys are the requested sizes of the image and the values are the URLs to those images.

e.g.

import image from './image.jpeg?sizes=200w+800w';
 
/*
image.sources => {
  '200w': 'xxx.jpg',
  '800w': 'xxx.jpg',
}
*/

placeholder

Only available if the option placeholder is present in either the loader configuration or the resource query.

This will generate an object with these keys:

  • url:
    • if the lightweight option is present, a tiny version of the image, encoded in base64.
    • otherwise, that tiny image but wrapped inside a SVG which applies a blur filter on the image.
  • color: an array containing the rgba color representing the most ubiquitous color of the image. [r, g, b, a]
  • ratio: the width to height ratio of the image.

e.g.

import image from './image.jpeg?sizes=200w+800w&placeholder';
/*
image.placeholder => {
  url: 'data:image/svg+xml;base64,...',
  color: [198, 123, 87, 1],
  ratio: 1.587302
}
*/

check the placeholder example for a possible use case.

Note: You can use placeholder on its own, without specifying sizes.