feat: add support for precompiled JSX

.changeset/dull-jobs-sin.md

@@ -0,0 +1,14 @@
+---
+'preact-render-to-string': minor
+---
+
+Add support for precompiled JSX transform, see https://deno.com/blog/v1.38#fastest-jsx-transform. Compared to traditional JSX transforms, the precompiled JSX transform tries to pre-serialize as much of the JSX as possible. That way less objects need to be created and serialized which relieves a lot of GC pressure.
+
+```jsx
+// input
+<div class="foo">hello</div>;
+
+// output
+const tpl = [`<div class="foo">hello</div>`];
+jsxTemplate(tpl);
+```

src/index.js

@@ -182,9 +182,36 @@ function _renderToString(vnode, context, isSvgMode, selectValue, parent) {
 	// Invoke rendering on Components
 	if (typeof type === 'function') {
 		if (type === Fragment) {
-			// Fragments are the least used components of core that's why
-			// branching here for comments has the least effect on perf.
-			if (props.UNSTABLE_comment) {
+			// Serialized precompiled JSX.
+			if (props.tpl) {
+				let out = '';
+				for (let i = 0; i < props.tpl.length; i++) {
+					out += props.tpl[i];
+
+					if (props.exprs && i < props.exprs.length) {
+						const value = props.exprs[i];
+						if (value == null) continue;
+
+						// Check if we're dealing with a vnode
+						if (typeof value === 'object' && value.constructor === undefined) {
+							out += _renderToString(
+								value,
+								context,
+								isSvgMode,
+								selectValue,
+								vnode
+							);
+						} else {
+							// Values are pre-escaped by the JSX transform
+							out += props.exprs[i];
+						}
+					}
+				}
+
+				return out;
+			} else if (props.UNSTABLE_comment) {
+				// Fragments are the least used components of core that's why
+				// branching here for comments has the least effect on perf.
 				return '<!--' + encodeEntities(props.UNSTABLE_comment || '') + '-->';
 			}
 

test/render.test.js

@@ -1650,4 +1650,28 @@ describe('render', () => {
 			}
 		});
 	});
+
+	describe('precompiled JSX', () => {
+		it('should render template', () => {
+			let vnode = <Fragment tpl={['<div>foo</div>']} exprs={[]} />;
+			let rendered = render(vnode);
+			expect(rendered).to.equal('<div>foo</div>');
+		});
+
+		it('should render template with attribute expressions', () => {
+			let vnode = (
+				<Fragment tpl={['<div ', '>foo</div>']} exprs={['class="foo"']} />
+			);
+			let rendered = render(vnode);
+			expect(rendered).to.equal('<div class="foo">foo</div>');
+		});
+
+		it('should render template with child expressions', () => {
+			let vnode = (
+				<Fragment tpl={['<div>foo', '</div>']} exprs={[<span>bar</span>]} />
+			);
+			let rendered = render(vnode);
+			expect(rendered).to.equal('<div>foo<span>bar</span></div>');
+		});
+	});
 });