Faster SSR (#5701)

Co-authored-by: Conduitry <git@chor.date>
Co-authored-by: Ben McCann <322311+benmccann@users.noreply.github.com>

src/compiler/compile/render_ssr/handlers/shared/get_attribute_value.ts

@@ -9,7 +9,7 @@ export function get_class_attribute_value(attribute: Attribute): ESTreeExpressio
 	// handle special case — `class={possiblyUndefined}` with scoped CSS
 	if (attribute.chunks.length === 2 && (attribute.chunks[1] as Text).synthetic) {
 		const value = (attribute.chunks[0] as Expression).node;
-		return x`@escape(@null_to_empty(${value})) + "${(attribute.chunks[1] as Text).data}"`;
+		return x`@escape(@null_to_empty(${value}), true) + "${(attribute.chunks[1] as Text).data}"`;
 	}
 
 	return get_attribute_value(attribute);
@@ -22,7 +22,7 @@ export function get_attribute_value(attribute: Attribute): ESTreeExpression {
 		.map((chunk) => {
 			return chunk.type === 'Text'
 				? string_literal(chunk.data.replace(/"/g, '"')) as ESTreeExpression
-				: x`@escape(${chunk.node})`;
+				: x`@escape(${chunk.node}, true)`;
 		})
 		.reduce((lhs, rhs) => x`${lhs} + ${rhs}`);
 }

src/runtime/internal/ssr.ts

@@ -69,20 +69,34 @@ export function merge_ssr_styles(style_attribute, style_directive) {
 	return style_object;
 }
 
-export const escaped = {
-	'"': '"',
-	"'": ''',
-	'&': '&',
-	'<': '&lt;',
-	'>': '&gt;'
-};
-
-export function escape(html) {
-	return String(html).replace(/["'&<>]/g, match => escaped[match]);
+const ATTR_REGEX = /[&"]/g;
+const CONTENT_REGEX = /[&<]/g;
+
+/**
+ * Note: this method is performance sensitive and has been optimized
+ * https://github.com/sveltejs/svelte/pull/5701
+ */
+export function escape(value: unknown, is_attr = false) {
+	const str = String(value);
+
+	const pattern = is_attr ? ATTR_REGEX : CONTENT_REGEX;
+	pattern.lastIndex = 0;
+
+	let escaped = '';
+	let last = 0;
+
+  while (pattern.test(str)) {
+    const i = pattern.lastIndex - 1;
+    const ch = str[i];
+    escaped += str.substring(last, i) + (ch === '&' ? '&amp;' : (ch === '"' ? '&quot;' : '&lt;'));
+    last = i + 1;
+  }
+
+	return escaped + str.substring(last);
 }
 
 export function escape_attribute_value(value) {
-	return typeof value === 'string' ? escape(value) : value;
+	return typeof value === 'string' ? escape(value, true) : value;
 }
 
 export function escape_object(obj) {