feat: add dynamic image optimization

part of #241
closes #9787

This adds image optimization through a new $app/images import. It's deliberately low level: The only export is getImage which you pass an image src and it returns an object containing src and srcset (possibly more?) values which you spread on an img tag.

In order to use this you need to define a path to a loader in kit.config.images. The loader takes the original img src and a width and returns a URL pointing to the optimized image. You can also modify the number of sizes and trusted domains.

packages/adapter-vercel/image-loader.d.ts

@@ -0,0 +1,4 @@
+/**
+ * https://vercel.com/docs/concepts/image-optimization
+ */
+export default function loader(src: string, width: number, options?: { quality?: number }): string;

packages/adapter-vercel/image-loader.js

@@ -0,0 +1,49 @@
+// https://vercel.com/docs/concepts/image-optimization
+
+/**
+ * @param {string} src
+ * @param {number} width
+ * @param {{ quality?: number }} [options]
+ */
+export default function loader(src, width, options) {
+	const url = new URL(src, 'http://n'); // If the base is a relative URL, we need to add a dummy host to the URL
+	if (url.pathname === '/_vercel/image') {
+		set_param(url, 'w', width);
+		set_param(url, 'q', options?.quality ?? 75, false);
+	} else {
+		url.pathname = `/_vercel/image`;
+		set_param(url, 'url', src);
+		set_param(url, 'w', width);
+		set_param(url, 'q', options?.quality ?? 75);
+	}
+	return src === url.href ? url.href : relative_url(url);
+}
+
+/**
+ * @param {URL} url
+ */
+function relative_url(url) {
+	const { pathname, search } = url;
+	return `${pathname}${search}`;
+}
+/**
+ * @param {URL} url
+ * @param {string} param
+ * @param {any} value
+ * @param {boolean} [override]
+ */
+function set_param(url, param, value, override = true) {
+	if (value === undefined) {
+		return;
+	}
+
+	if (value === null) {
+		if (override || url.searchParams.has(param)) {
+			url.searchParams.delete(param);
+		}
+	} else {
+		if (override || !url.searchParams.has(param)) {
+			url.searchParams.set(param, value);
+		}
+	}
+}

packages/adapter-vercel/index.d.ts

@@ -1,7 +1,16 @@
 import { Adapter } from '@sveltejs/kit';
 import './ambient.js';
 
-export default function plugin(config?: Config): Adapter;
+export default function plugin(
+	config?: Config & {
+		/**
+		 * Enable or disable Vercel's image optimization. This is enabled by default if you have
+		 * defined the Vercel loader in your `svelte.config.js` file, else disabled by default.
+		 * https://vercel.com/docs/concepts/image-optimization
+		 */
+		images?: boolean;
+	}
+): Adapter;
 
 export interface ServerlessConfig {
 	/**

packages/adapter-vercel/index.js

@@ -407,8 +407,22 @@ function static_vercel_config(builder) {
 		overrides[page.file] = { path: overrides_path };
 	}
 
+	/** @type {Record<string, any> | undefined} */
+	let images = undefined;
+	const img_config = builder.config.kit.images;
+	if (config.images || img_config.loader === '@sveltejs/adapter-vercel/image-loader') {
+		images = {
+			sizes: img_config.sizes,
+			domains: img_config.domains,
+			// TODO should we expose the following and some other optional options through the adapter?
+			formats: ['image/avif', 'image/webp'],
+			minimumCacheTTL: 300
+		};
+	}
+
 	return {
 		version: 3,
+		images,
 		routes: [
 			...prerendered_redirects,
 			{

packages/adapter-vercel/package.json

@@ -15,6 +15,10 @@
 			"types": "./index.d.ts",
 			"import": "./index.js"
 		},
+		"./image-loader": {
+			"types": "./image-loader.d.ts",
+			"import": "./image-loader.js"
+		},
 		"./package.json": "./package.json"
 	},
 	"types": "index.d.ts",

packages/kit/scripts/generate-dts.js

@@ -10,6 +10,7 @@ createBundle({
 		'@sveltejs/kit/vite': 'src/exports/vite/index.js',
 		'$app/environment': 'src/runtime/app/environment.js',
 		'$app/forms': 'src/runtime/app/forms.js',
+		'$app/images': 'src/runtime/app/images.js',
 		'$app/navigation': 'src/runtime/app/navigation.js',
 		'$app/paths': 'src/runtime/app/paths.js',
 		'$app/stores': 'src/runtime/app/stores.js'

packages/kit/src/core/config/options.js

@@ -138,6 +138,12 @@ const options = object(
 				errorTemplate: string(join('src', 'error.html'))
 			}),
 
+			images: object({
+				domains: string_array([]),
+				loader: string(null),
+				sizes: number_array([640, 828, 1200, 1920, 3840])
+			}),
+
 			inlineStyleThreshold: number(0),
 
 			moduleExtensions: string_array(['.js', '.ts']),
@@ -354,6 +360,20 @@ function string(fallback, allow_empty = true) {
 	});
 }
 
+/**
+ * @param {number[] | undefined} [fallback]
+ * @returns {Validator}
+ */
+function number_array(fallback) {
+	return validate(fallback, (input, keypath) => {
+		if (!Array.isArray(input) || input.some((value) => typeof value !== 'number')) {
+			throw new Error(`${keypath} must be an array of numbers, if specified`);
+		}
+
+		return input;
+	});
+}
+
 /**
  * @param {string[] | undefined} [fallback]
  * @returns {Validator}

packages/kit/src/exports/public.d.ts

@@ -430,6 +430,31 @@ export interface KitConfig {
 		 */
 		errorTemplate?: string;
 	};
+	/**
+	 * Image optimization configuration
+	 */
+	images?: {
+		/**
+		 * Path to a a file that contains a loader that will be used to generate the an image URL out of the given source and width.
+		 * It optionally also takes third parameter for options.
+		 *
+		 * ```js
+		 * export default function loader(src, width, opts) {
+		 *  return `https://example.com/${src}?w=${width}&q=${opts.quality || 75}`;
+		 * }
+		 * ```
+		 */
+		loader?: string;
+		/**
+		 * Which srcset sizes to generate
+		 * @default [640, 828, 1200, 1920, 3840]
+		 */
+		sizes?: number[];
+		/**
+		 * Which external domains to trust when optimizing images
+		 */
+		domains?: string[];
+	};
 	/**
 	 * Inline CSS inside a `<style>` block at the head of the HTML. This option is a number that specifies the maximum length of a CSS file to be inlined. All CSS files needed for the page and smaller than this value are merged and inlined in a `<style>` block.
 	 *

packages/kit/src/exports/vite/index.js

@@ -441,6 +441,19 @@ function kit({ svelte_config }) {
 						}
 					`;
 				}
+
+				case '\0__sveltekit/images': {
+					const { images } = svelte_config.kit;
+					const loader = images.loader
+						? `export { default as loader } from '${images.loader}';`
+						: 'export function loader(src) { console.warn("No image loader in kit.config.kit.images.loader set, images will not be optimized."); return src; }';
+
+					return dedent`
+						export const sizes = ${JSON.stringify(images.sizes)};
+						export const domains = ${JSON.stringify(images.domains)};
+						${loader}
+					`;
+				}
 			}
 		}
 	};

packages/kit/src/runtime/app/images.js

@@ -0,0 +1,47 @@
+import { DEV } from 'esm-env';
+import { sizes, loader, domains } from '__sveltekit/images';
+
+/**
+ * @param {string} src
+ * @param {any} [options]
+ * @returns {{ src: string, srcset?: string }}
+ */
+export function getImage(src, options) {
+	if (DEV) {
+		if (!matches_domain(src)) {
+			console.warn(
+				`$app/images: Image src '${src}' does not match any of the allowed domains and will therefore not be optimized.`
+			);
+		}
+		return { srcset: src, src };
+	}
+
+	if (!matches_domain(src)) {
+		return { src };
+	}
+
+	const srcset = sizes
+		.map((size) => {
+			const url = loader(src, size, options);
+			const w = size + 'w';
+			return `${url} ${w}`;
+		})
+		.join(', ');
+	const _src = loader(src, sizes[sizes.length - 1], options);
+
+	// Order of attributes is important here as they are set in this order
+	// and having src before srcset would result in a flicker
+	return { srcset, src: _src };
+}
+
+/**
+ * @param {string} src
+ */
+function matches_domain(src) {
+	const url = new URL(src, 'http://n'); // if src is protocol relative, use dummy domain
+	if (url.href === src) {
+		return domains.some((domain) => url.hostname === domain);
+	} else {
+		return true; // relative urls are always ok
+	}
+}

packages/kit/src/types/ambient.d.ts

@@ -106,3 +106,10 @@ declare module '__sveltekit/paths' {
 	export function override(paths: { base: string; assets: string }): void;
 	export function set_assets(path: string): void;
 }
+
+/** Internal version of $app/images */
+declare module '__sveltekit/images' {
+	export let sizes: number[];
+	export let loader: (url: string, size: number, opts?: any) => string;
+	export let domains: string[];
+}