feat: never quote single attribute expressions in Svelte 5

README.md

@@ -99,12 +99,16 @@ More strict HTML syntax: Quotes in attributes and no self-closing DOM elements (
 
 > In version 2 this overruled `svelteAllowShorthand`, which is no longer the case.
 
+> In Svelte 5, attributes are never quoted, because this will mean "stringify this attribute value" in a future Svelte version
+
 Example:
 
 <!-- prettier-ignore -->
 ```html
-<!-- svelteStrictMode: true -->
+<!-- svelteStrictMode: true (Svelte 3 and 4) -->
 <div foo="{bar}"></div>
+<!-- svelteStrictMode: true (Svelte 5) -->
+<div foo={bar}></div>
 
 <!-- svelteStrictMode: false -->
 <div foo={bar} />

src/embed.ts

@@ -78,7 +78,13 @@ export function embed(path: FastPath, _options: Options) {
     const parent: Node = path.getParentNode();
     const printJsExpression = () =>
         (parent as any).expression
-            ? printJS(parent, options.svelteStrictMode ?? false, false, false, 'expression')
+            ? printJS(
+                  parent,
+                  (options.svelteStrictMode && !options._svelte_is5Plus) ?? false,
+                  false,
+                  false,
+                  'expression',
+              )
             : undefined;
     const printSvelteBlockJS = (name: string) => printJS(parent, false, true, false, name);
 
@@ -110,7 +116,13 @@ export function embed(path: FastPath, _options: Options) {
             }
             break;
         case 'Element':
-            printJS(parent, options.svelteStrictMode ?? false, false, false, 'tag');
+            printJS(
+                parent,
+                (options.svelteStrictMode && !options._svelte_is5Plus) ?? false,
+                false,
+                false,
+                'tag',
+            );
             break;
         case 'MustacheTag':
             printJS(parent, isInsideQuotedAttribute(path, options), false, false, 'expression');

src/index.ts

@@ -5,6 +5,7 @@ import { ASTNode } from './print/nodes';
 import { embed, getVisitorKeys } from './embed';
 import { snipScriptAndStyleTagContent } from './lib/snipTagContent';
 import { parse, VERSION } from 'svelte/compiler';
+import { ParserOptions } from './options';
 
 const babelParser = prettierPluginBabel.parsers.babel;
 const typescriptParser = prettierPluginBabel.parsers['babel-ts']; // TODO use TypeScript parser in next major?
@@ -46,7 +47,7 @@ export const parsers: Record<string, Parser> = {
                 throw err;
             }
         },
-        preprocess: (text, options) => {
+        preprocess: (text, options: ParserOptions) => {
             const result = snipScriptAndStyleTagContent(text);
             text = result.text.trim();
             // Prettier sets the preprocessed text as the originalText in case
@@ -56,7 +57,8 @@ export const parsers: Record<string, Parser> = {
             // Therefore we do it ourselves here.
             options.originalText = text;
             // Only Svelte 5 can have TS in the template
-            (options as any)._svelte_ts = isSvelte5Plus && result.isTypescript;
+            options._svelte_ts = isSvelte5Plus && result.isTypescript;
+            options._svelte_is5Plus = isSvelte5Plus;
             return text;
         },
         locStart,

src/options.ts

@@ -3,6 +3,15 @@ import { SortOrder, PluginConfig } from '..';
 
 export interface ParserOptions<T = any> extends PrettierParserOptions<T>, Partial<PluginConfig> {
     _svelte_ts?: boolean;
+    _svelte_asFunction?: boolean;
+    /**
+     * Used for
+     * - deciding what quote behavior to use in the printer:
+     *   A future version of Svelte treats quoted expressions as strings, so never use quotes in that case.
+     *   Since Svelte 5 does still treat them equally, it's safer to remove quotes in all cases and in a future
+     *   version of this plugin instead leave it up to the user to decide.
+     */
+    _svelte_is5Plus?: boolean;
 }
 
 function makeChoice(choice: string) {

src/print/index.ts

@@ -26,7 +26,6 @@ import {
     isIgnoreEndDirective,
     isIgnoreStartDirective,
     isInlineElement,
-    isInsideQuotedAttribute,
     isLoneMustacheTag,
     isNodeSupportedLanguage,
     isNodeTopLevelHTML,
@@ -89,7 +88,8 @@ export function print(path: FastPath, options: ParserOptions, print: PrintFn): D
         return printTopLevelParts(n, options, path, print);
     }
 
-    const [open, close] = options.svelteStrictMode ? ['"{', '}"'] : ['{', '}'];
+    const [open, close] =
+        options.svelteStrictMode && !options._svelte_is5Plus ? ['"{', '}"'] : ['{', '}'];
     const printJsExpression = () => [open, printJS(path, print, 'expression'), close];
     const node = n as Node;
 
@@ -438,18 +438,17 @@ export function print(path: FastPath, options: ParserOptions, print: PrintFn): D
             if (isOrCanBeConvertedToShorthand(node)) {
                 if (options.svelteAllowShorthand) {
                     return ['{', node.name, '}'];
-                } else if (options.svelteStrictMode) {
-                    return [node.name, '="{', node.name, '}"'];
                 } else {
-                    return [node.name, '={', node.name, '}'];
+                    return [node.name, `=${open}`, node.name, close];
                 }
             } else {
                 if (node.value === true) {
                     return [node.name];
                 }
 
                 const quotes =
-                    !isLoneMustacheTag(node.value) || (options.svelteStrictMode ?? false);
+                    !isLoneMustacheTag(node.value) ||
+                    ((options.svelteStrictMode && !options._svelte_is5Plus) ?? false);
                 const attrNodeValue = printAttributeNodeValue(path, print, quotes, node);
                 if (quotes) {
                     return [node.name, '=', '"', attrNodeValue, '"'];
@@ -649,14 +648,13 @@ export function print(path: FastPath, options: ParserOptions, print: PrintFn): D
             if (isOrCanBeConvertedToShorthand(node) || node.value === true) {
                 if (options.svelteAllowShorthand) {
                     return [...prefix];
-                } else if (options.svelteStrictMode) {
-                    return [...prefix, '="{', node.name, '}"'];
                 } else {
-                    return [...prefix, '={', node.name, '}'];
+                    return [...prefix, `=${open}`, node.name, close];
                 }
             } else {
                 const quotes =
-                    !isLoneMustacheTag(node.value) || (options.svelteStrictMode ?? false);
+                    !isLoneMustacheTag(node.value) ||
+                    ((options.svelteStrictMode && !options._svelte_is5Plus) ?? false);
                 const attrNodeValue = printAttributeNodeValue(path, print, quotes, node);
                 if (quotes) {
                     return [...prefix, '=', '"', attrNodeValue, '"'];

src/print/node-helpers.ts

@@ -534,7 +534,8 @@ export function isInsideQuotedAttribute(path: FastPath, options: ParserOptions):
     return stack.some(
         (node) =>
             node.type === 'Attribute' &&
-            (!isLoneMustacheTag(node.value) || options.svelteStrictMode),
+            (!isLoneMustacheTag(node.value) ||
+                (options.svelteStrictMode && !options._svelte_is5Plus)),
     );
 }
 

test/formatting/index.ts

@@ -1,8 +1,11 @@
 import test from 'ava';
 import { readdirSync, readFileSync, existsSync } from 'fs';
 import { format } from 'prettier';
+import { VERSION } from 'svelte/compiler';
 import * as SveltePlugin from '../../src';
 
+const isSvelte5Plus = Number(VERSION.split('.')[0]) >= 5;
+
 let dirs = readdirSync('test/formatting/samples');
 const printerFilesHaveOnly = readdirSync('test/printer/samples').some(
     (f) => f.endsWith('.only.html') || f.endsWith('.only.md'),
@@ -26,6 +29,9 @@ for (const dir of dirs) {
     ).replace(/\r?\n/g, '\n');
     const options = readOptions(`test/formatting/samples/${dir}/options.json`);
 
+    // Tests attribute quoting changes, which are different in Svelte 5
+    if (dir.endsWith('strict-mode-true') && isSvelte5Plus) continue;
+
     test(`formatting: ${dir}`, async (t) => {
         let onTestCompleted;
 

test/printer/index.ts

@@ -1,8 +1,11 @@
 import test from 'ava';
 import { readdirSync, readFileSync, existsSync } from 'fs';
 import { format } from 'prettier';
+import { VERSION } from 'svelte/compiler';
 import * as SveltePlugin from '../../src';
 
+const isSvelte5Plus = Number(VERSION.split('.')[0]) >= 5;
+
 let files = readdirSync('test/printer/samples').filter(
     (name) => name.endsWith('.html') || name.endsWith('.md'),
 );
@@ -24,6 +27,9 @@ for (const file of files) {
         `test/printer/samples/${file.replace('.only', '').replace(`.${ending}`, '.options.json')}`,
     );
 
+    // Tests attribute quoting changes, which are different in Svelte 5
+    if (file.endsWith('attribute-quoted.html') && isSvelte5Plus) continue;
+
     test(`printer: ${file.slice(0, file.length - `.${ending}`.length)}`, async (t) => {
         const actualOutput = await format(input, {
             parser: ending === 'html' ? 'svelte' : 'markdown',