chore: simpler fallback values (#12788)

* DRY

* value_or_fallback_async doesnt do anything

* rename value_or_fallback to fallback

* DRY

* DRY

* make lazy the exception

* changeset

Author: Rich-Harris

.changeset/witty-frogs-cheat.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+chore: simpler fallback values

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

@@ -2,7 +2,7 @@
 /** @import { Binding } from '#compiler' */
 /** @import { Context } from '../types.js' */
 /** @import { Scope } from '../../../scope.js' */
-import { extract_paths, is_expression_async } from '../../../../utils/ast.js';
+import { build_fallback, extract_paths } from '../../../../utils/ast.js';
 import * as b from '../../../../utils/builders.js';
 import { get_rune } from '../../../scope.js';
 import { walk } from 'zimmerframe';
@@ -96,9 +96,7 @@ export function VariableDeclaration(node, context) {
 						const name = /** @type {Identifier} */ (path.node).name;
 						const binding = /** @type {Binding} */ (context.state.scope.get(name));
 						const prop = b.member(b.id('$$props'), b.literal(binding.prop_alias ?? name), true);
-						declarations.push(
-							b.declarator(path.node, b.call('$.value_or_fallback', prop, b.thunk(value)))
-						);
+						declarations.push(b.declarator(path.node, build_fallback(prop, value)));
 					}
 					continue;
 				}
@@ -114,9 +112,7 @@ export function VariableDeclaration(node, context) {
 				let init = prop;
 				if (declarator.init) {
 					const default_value = /** @type {Expression} */ (context.visit(declarator.init));
-					init = is_expression_async(default_value)
-						? b.await(b.call('$.value_or_fallback_async', prop, b.thunk(default_value, true)))
-						: b.call('$.value_or_fallback', prop, b.thunk(default_value));
+					init = build_fallback(prop, default_value);
 				}
 
 				declarations.push(b.declarator(declarator.id, init));

packages/svelte/src/compiler/utils/ast.js

@@ -368,12 +368,7 @@ function _extract_paths(assignments = [], param, expression, update_expression,
 
 		case 'AssignmentPattern': {
 			/** @type {DestructuredAssignment['expression']} */
-			const fallback_expression = (object) =>
-				is_expression_async(param.right)
-					? b.await(
-							b.call('$.value_or_fallback_async', expression(object), b.thunk(param.right, true))
-						)
-					: b.call('$.value_or_fallback', expression(object), b.thunk(param.right));
+			const fallback_expression = (object) => build_fallback(expression(object), param.right);
 
 			if (param.left.type === 'Identifier') {
 				assignments.push({
@@ -549,3 +544,22 @@ export function is_expression_async(expression) {
 			return false;
 	}
 }
+
+/**
+ *
+ * @param {ESTree.Expression} expression
+ * @param {ESTree.Expression} fallback
+ */
+export function build_fallback(expression, fallback) {
+	if (is_simple_expression(fallback)) {
+		return b.call('$.fallback', expression, fallback);
+	}
+
+	if (fallback.type === 'AwaitExpression' && is_simple_expression(fallback.argument)) {
+		return b.await(b.call('$.fallback', expression, fallback.argument));
+	}
+
+	return is_expression_async(fallback)
+		? b.await(b.call('$.fallback', expression, b.thunk(fallback, true), b.true))
+		: b.call('$.fallback', expression, b.thunk(fallback), b.true);
+}

packages/svelte/src/internal/client/index.js

@@ -134,8 +134,6 @@ export {
 	untrack,
 	update,
 	update_pre,
-	value_or_fallback,
-	value_or_fallback_async,
 	exclude_from_object,
 	pop,
 	push,
@@ -164,7 +162,7 @@ export {
 	$document as document
 } from './dom/operations.js';
 export { snapshot } from '../shared/clone.js';
-export { noop } from '../shared/utils.js';
+export { noop, fallback } from '../shared/utils.js';
 export {
 	invalid_default_snippet,
 	validate_dynamic_element_tag,

packages/svelte/src/internal/client/runtime.js

@@ -1003,26 +1003,6 @@ export function exclude_from_object(obj, keys) {
 	return result;
 }
 
-/**
- * @template V
- * @param {V} value
- * @param {() => V} fallback lazy because could contain side effects
- * @returns {V}
- */
-export function value_or_fallback(value, fallback) {
-	return value === undefined ? fallback() : value;
-}
-
-/**
- * @template V
- * @param {V} value
- * @param {() => Promise<V>} fallback lazy because could contain side effects
- * @returns {Promise<V>}
- */
-export async function value_or_fallback_async(value, fallback) {
-	return value === undefined ? fallback() : value;
-}
-
 /**
  * @param {Record<string, unknown>} props
  * @param {any} runes

packages/svelte/src/internal/server/index.js

@@ -383,26 +383,6 @@ export function unsubscribe_stores(store_values) {
 	}
 }
 
-/**
- * @template V
- * @param {V} value
- * @param {() => V} fallback lazy because could contain side effects
- * @returns {V}
- */
-export function value_or_fallback(value, fallback) {
-	return value === undefined ? fallback() : value;
-}
-
-/**
- * @template V
- * @param {V} value
- * @param {() => Promise<V>} fallback lazy because could contain side effects
- * @returns {Promise<V>}
- */
-export async function value_or_fallback_async(value, fallback) {
-	return value === undefined ? fallback() : value;
-}
-
 /**
  * @param {Payload} payload
  * @param {void | ((payload: Payload, props: Record<string, unknown>) => void)} slot_fn
@@ -536,6 +516,8 @@ export { push_element, pop_element } from './dev.js';
 
 export { snapshot } from '../shared/clone.js';
 
+export { fallback } from '../shared/utils.js';
+
 export {
 	invalid_default_snippet,
 	validate_dynamic_element_tag,

packages/svelte/src/internal/shared/utils.js

@@ -46,3 +46,18 @@ export function run_all(arr) {
 		arr[i]();
 	}
 }
+
+/**
+ * @template V
+ * @param {V} value
+ * @param {V | (() => V)} fallback
+ * @param {boolean} [lazy]
+ * @returns {V}
+ */
+export function fallback(value, fallback, lazy = false) {
+	return value === undefined
+		? lazy
+			? /** @type {() => V} */ (fallback)()
+			: /** @type {V} */ (fallback)
+		: value;
+}