chore: remove unnecessary `?? ''` on some expressions (#15287)

* no `?? ''` on some expressions

* changeset

* delete operator returns boolean

* inverted conditions are a lil confusing

* no need for else after return

* simplify condition

* may as well add special handling for undefined while we're here

* use normal string literal when there are no values

* omit assignment when there is no text content

---------

Co-authored-by: Rich Harris <rich.harris@vercel.com>

.changeset/serious-glasses-kiss.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+chore: remove unnecessary `?? ''` on some expressions

packages/svelte/src/compiler/phases/3-transform/client/visitors/RegularElement.js

@@ -364,15 +364,14 @@ export function RegularElement(node, context) {
 		trimmed.some((node) => node.type === 'ExpressionTag');
 
 	if (use_text_content) {
-		child_state.init.push(
-			b.stmt(
-				b.assignment(
-					'=',
-					b.member(context.state.node, 'textContent'),
-					build_template_chunk(trimmed, context.visit, child_state).value
-				)
-			)
-		);
+		const { value } = build_template_chunk(trimmed, context.visit, child_state);
+		const empty_string = value.type === 'Literal' && value.value === '';
+
+		if (!empty_string) {
+			child_state.init.push(
+				b.stmt(b.assignment('=', b.member(context.state.node, 'textContent'), value))
+			);
+		}
 	} else {
 		/** @type {Expression} */
 		let arg = context.state.node;

packages/svelte/src/compiler/phases/3-transform/client/visitors/shared/utils.js

@@ -101,11 +101,15 @@ export function build_template_chunk(
 
 		if (node.type === 'Text') {
 			quasi.value.cooked += node.data;
-		} else if (node.type === 'ExpressionTag' && node.expression.type === 'Literal') {
+		} else if (node.expression.type === 'Literal') {
 			if (node.expression.value != null) {
 				quasi.value.cooked += node.expression.value + '';
 			}
-		} else {
+		} else if (
+			node.expression.type !== 'Identifier' ||
+			node.expression.name !== 'undefined' ||
+			state.scope.get('undefined')
+		) {
 			let value = memoize(
 				/** @type {Expression} */ (visit(node.expression, state)),
 				node.metadata.expression
@@ -117,31 +121,33 @@ export function build_template_chunk(
 				// If we have a single expression, then pass that in directly to possibly avoid doing
 				// extra work in the template_effect (instead we do the work in set_text).
 				return { value, has_state };
-			} else {
-				// add `?? ''` where necessary (TODO optimise more cases)
-				if (
-					value.type === 'LogicalExpression' &&
-					value.right.type === 'Literal' &&
-					(value.operator === '??' || value.operator === '||')
-				) {
-					// `foo ?? null` -=> `foo ?? ''`
-					// otherwise leave the expression untouched
-					if (value.right.value === null) {
-						value = { ...value, right: b.literal('') };
-					}
-				} else if (
-					state.analysis.props_id &&
-					value.type === 'Identifier' &&
-					value.name === state.analysis.props_id.name
-				) {
-					// do nothing ($props.id() is never null/undefined)
-				} else {
-					value = b.logical('??', value, b.literal(''));
+			}
+
+			if (
+				value.type === 'LogicalExpression' &&
+				value.right.type === 'Literal' &&
+				(value.operator === '??' || value.operator === '||')
+			) {
+				// `foo ?? null` -=> `foo ?? ''`
+				// otherwise leave the expression untouched
+				if (value.right.value === null) {
+					value = { ...value, right: b.literal('') };
 				}
+			}
+
+			const is_defined =
+				value.type === 'BinaryExpression' ||
+				(value.type === 'UnaryExpression' && value.operator !== 'void') ||
+				(value.type === 'LogicalExpression' && value.right.type === 'Literal') ||
+				(value.type === 'Identifier' && value.name === state.analysis.props_id?.name);
 
-				expressions.push(value);
+			if (!is_defined) {
+				// add `?? ''` where necessary (TODO optimise more cases)
+				value = b.logical('??', value, b.literal(''));
 			}
 
+			expressions.push(value);
+
 			quasi = b.quasi('', i + 1 === values.length);
 			quasis.push(quasi);
 		}
@@ -151,7 +157,10 @@ export function build_template_chunk(
 		quasi.value.raw = sanitize_template_string(/** @type {string} */ (quasi.value.cooked));
 	}
 
-	const value = b.template(quasis, expressions);
+	const value =
+		expressions.length > 0
+			? b.template(quasis, expressions)
+			: b.literal(/** @type {string} */ (quasi.value.cooked));
 
 	return { value, has_state };
 }