feat: add partial evaluation (#15494)

* feat: add partial evaluation

* fix

* tweak

* more

* more

* evaluate stuff in template

* update test

* SSR

* unused

* changeset

* remove TODO

* Apply suggestions from code review

Co-authored-by: Simon H <[email protected]>

* allow unknown operators

* use blocks and block-scoping in switch statement

---------

Co-authored-by: Simon H <[email protected]>

Author: Rich-Harris

.changeset/selfish-onions-begin.md

@@ -0,0 +1,5 @@
+---
+'svelte': minor
+---
+
+feat: partially evaluate certain expressions

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

@@ -685,14 +685,13 @@ function build_element_special_value_attribute(element, node_id, attribute, cont
 			: value
 	);
 
+	const evaluated = context.state.scope.evaluate(value);
+	const assignment = b.assignment('=', b.member(node_id, '__value'), value);
+
 	const inner_assignment = b.assignment(
 		'=',
 		b.member(node_id, 'value'),
-		b.conditional(
-			b.binary('==', b.null, b.assignment('=', b.member(node_id, '__value'), value)),
-			b.literal(''), // render null/undefined values as empty string to support placeholder options
-			value
-		)
+		evaluated.is_defined ? assignment : b.logical('??', assignment, b.literal(''))
 	);
 
 	const update = b.stmt(

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

@@ -89,21 +89,21 @@ export function build_template_chunk(
 				}
 			}
 
-			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);
-
-			if (!is_defined) {
-				// add `?? ''` where necessary (TODO optimise more cases)
-				value = b.logical('??', value, b.literal(''));
-			}
+			const evaluated = state.scope.evaluate(value);
 
-			expressions.push(value);
+			if (evaluated.is_known) {
+				quasi.value.cooked += evaluated.value + '';
+			} else {
+				if (!evaluated.is_defined) {
+					// add `?? ''` where necessary
+					value = b.logical('??', value, b.literal(''));
+				}
 
-			quasi = b.quasi('', i + 1 === values.length);
-			quasis.push(quasi);
+				expressions.push(value);
+
+				quasi = b.quasi('', i + 1 === values.length);
+				quasis.push(quasi);
+			}
 		}
 	}
 

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

@@ -44,15 +44,17 @@ export function process_children(nodes, { visit, state }) {
 			if (node.type === 'Text' || node.type === 'Comment') {
 				quasi.value.cooked +=
 					node.type === 'Comment' ? `<!--${node.data}-->` : escape_html(node.data);
-			} else if (node.type === 'ExpressionTag' && node.expression.type === 'Literal') {
-				if (node.expression.value != null) {
-					quasi.value.cooked += escape_html(node.expression.value + '');
-				}
 			} else {
-				expressions.push(b.call('$.escape', /** @type {Expression} */ (visit(node.expression))));
+				const evaluated = state.scope.evaluate(node.expression);
+
+				if (evaluated.is_known) {
+					quasi.value.cooked += escape_html((evaluated.value ?? '') + '');
+				} else {
+					expressions.push(b.call('$.escape', /** @type {Expression} */ (visit(node.expression))));
 
-				quasi = b.quasi('', i + 1 === sequence.length);
-				quasis.push(quasi);
+					quasi = b.quasi('', i + 1 === sequence.length);
+					quasis.push(quasi);
+				}
 			}
 		}