feat: add naturalWidth and naturalHeight bindings (#7857)

Closes #7771
---------

Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>
Co-authored-by: Simon Holthausen <simon.holthausen@vercel.com>

site/content/docs/03-template-syntax.md

@@ -751,6 +751,22 @@ Videos additionally have readonly `videoWidth` and `videoHeight` bindings.
 ></video>
 ```
 
+##### Image element bindings
+
+---
+
+Image elements (`<img>`) have two readonly bindings:
+
+* `naturalWidth` (readonly) — the original width of the image, available after the image has loaded
+* `naturalHeight` (readonly) — the original height of the image, available after the image has loaded
+
+```sv
+<img
+	bind:naturalWidth
+	bind:naturalHeight
+></img>
+```
+
 ##### Block-level element bindings
 
 ---

src/compiler/compile/nodes/Binding.ts

@@ -22,7 +22,9 @@ const read_only_media_attributes = new Set([
 	'seeking',
 	'ended',
 	'videoHeight',
-	'videoWidth'
+	'videoWidth',
+	'naturalWidth',
+	'naturalHeight'
 ]);
 
 export default class Binding extends Node {

src/compiler/compile/nodes/Element.ts

@@ -918,6 +918,13 @@ export default class Element extends Node {
 				} else if (is_void(this.name)) {
 					return component.error(binding, compiler_errors.invalid_binding_on(binding.name, `void elements like <${this.name}>. Use a wrapper element instead`));
 				}
+			} else if (
+				name === 'naturalWidth' ||
+				name === 'naturalHeight'
+			) {
+				if (this.name !== 'img') {
+					return component.error(binding, compiler_errors.invalid_binding_element_with('<img>', name));
+				}
 			} else if (
 				name === 'textContent' ||
 				name === 'innerHTML'

src/compiler/compile/render_dom/wrappers/Element/index.ts

@@ -137,6 +137,10 @@ const events = [
 		event_names: ['toggle'],
 		filter: (node: Element, _name: string) =>
 			node.name === 'details'
+	},
+	{
+		event_names: ['load'],
+		filter: (_: Element, name: string) => name === 'naturalHeight' || name === 'naturalWidth'
 	}
 ];
 

test/js/samples/natural-dimension-binding/expected.js

@@ -0,0 +1,95 @@
+/* generated by Svelte vX.Y.Z */
+import {
+	SvelteComponent,
+	add_render_callback,
+	attr,
+	detach,
+	element,
+	init,
+	insert,
+	listen,
+	noop,
+	safe_not_equal,
+	set_data,
+	space,
+	src_url_equal,
+	text
+} from "svelte/internal";
+
+function create_fragment(ctx) {
+	let img;
+	let img_src_value;
+	let t0;
+	let t1;
+	let t2;
+	let t3;
+	let mounted;
+	let dispose;
+
+	return {
+		c() {
+			img = element("img");
+			t0 = space();
+			t1 = text(/*naturalWidth*/ ctx[0]);
+			t2 = text(" x ");
+			t3 = text(/*naturalHeight*/ ctx[1]);
+			if (!src_url_equal(img.src, img_src_value = "something.jpg")) attr(img, "src", img_src_value);
+			if (/*naturalWidth*/ ctx[0] === void 0 || /*naturalHeight*/ ctx[1] === void 0) add_render_callback(() => /*img_load_handler*/ ctx[2].call(img));
+		},
+		m(target, anchor) {
+			insert(target, img, anchor);
+			insert(target, t0, anchor);
+			insert(target, t1, anchor);
+			insert(target, t2, anchor);
+			insert(target, t3, anchor);
+
+			if (!mounted) {
+				dispose = listen(img, "load", /*img_load_handler*/ ctx[2]);
+				mounted = true;
+			}
+		},
+		p(ctx, [dirty]) {
+			if (dirty & /*naturalWidth*/ 1) set_data(t1, /*naturalWidth*/ ctx[0]);
+			if (dirty & /*naturalHeight*/ 2) set_data(t3, /*naturalHeight*/ ctx[1]);
+		},
+		i: noop,
+		o: noop,
+		d(detaching) {
+			if (detaching) detach(img);
+			if (detaching) detach(t0);
+			if (detaching) detach(t1);
+			if (detaching) detach(t2);
+			if (detaching) detach(t3);
+			mounted = false;
+			dispose();
+		}
+	};
+}
+
+function instance($$self, $$props, $$invalidate) {
+	let { naturalWidth = 0 } = $$props;
+	let { naturalHeight = 0 } = $$props;
+
+	function img_load_handler() {
+		naturalWidth = this.naturalWidth;
+		naturalHeight = this.naturalHeight;
+		$$invalidate(0, naturalWidth);
+		$$invalidate(1, naturalHeight);
+	}
+
+	$$self.$$set = $$props => {
+		if ('naturalWidth' in $$props) $$invalidate(0, naturalWidth = $$props.naturalWidth);
+		if ('naturalHeight' in $$props) $$invalidate(1, naturalHeight = $$props.naturalHeight);
+	};
+
+	return [naturalWidth, naturalHeight, img_load_handler];
+}
+
+class Component extends SvelteComponent {
+	constructor(options) {
+		super();
+		init(this, options, instance, create_fragment, safe_not_equal, { naturalWidth: 0, naturalHeight: 1 });
+	}
+}
+
+export default Component;

test/js/samples/natural-dimension-binding/input.svelte

@@ -0,0 +1,8 @@
+<script>
+  export let naturalWidth = 0;
+  export let naturalHeight = 0;
+</script>
+
+<img src="something.jpg" bind:naturalWidth bind:naturalHeight>
+
+{naturalWidth} x {naturalHeight}