Add class:list directive

packages/astro/src/runtime/server/index.ts

@@ -91,7 +91,7 @@ export function createComponent(cb: AstroComponentFactory) {
   return cb;
 }
 
-interface ExtractedHydration {
+interface ExtractedProps {
   hydration: {
     directive: string;
     value: string;
@@ -101,8 +101,8 @@ interface ExtractedHydration {
   props: Record<string | number, any>;
 }
 
-function extractHydrationDirectives(inputProps: Record<string | number, any>): ExtractedHydration {
-  let extracted: ExtractedHydration = {
+function extractDirectives(inputProps: Record<string | number, any>): ExtractedProps {
+  let extracted: ExtractedProps = {
     hydration: null,
     props: {},
   };
@@ -131,6 +131,9 @@ function extractHydrationDirectives(inputProps: Record<string | number, any>): E
           break;
         }
       }
+    } else if (key === 'class:list') {
+      // support "class" from an expression passed into a component (#782)
+      extracted.props[key.slice(0, -5)] = serializeListValue(value)
     } else {
       extracted.props[key] = value;
     }
@@ -200,7 +203,7 @@ export async function renderComponent(result: SSRResult, displayName: string, Co
     throw new Error(`Unable to render ${metadata.displayName} because it is ${Component}!\nDid you forget to import the component or is it possible there is a typo?`);
   }
 
-  const { hydration, props } = extractHydrationDirectives(_props);
+  const { hydration, props } = extractDirectives(_props);
   let html = '';
 
   if (hydration) {
@@ -287,6 +290,12 @@ export function addAttribute(value: any, key: string) {
   if (value == null || value === false) {
     return '';
   }
+
+  // support "class" from an expression passed into an element (#782)
+  if (key === 'class:list') {
+    return ` ${key.slice(0, -5)}="${serializeListValue(value)}"`;
+  }
+
   return ` ${key}="${value}"`;
 }
 
@@ -298,6 +307,33 @@ export function spreadAttributes(values: Record<any, any>) {
   return output;
 }
 
+function serializeListValue(value: any) {
+  const hash: Record<string, any> = {}
+
+  push(value)
+
+  return Object.keys(hash).join(' ');
+
+  function push(item: any) {
+    // push individual iteratables
+    if (item && typeof item.forEach === 'function') item.forEach(push)
+
+    // otherwise, push object value keys by truthiness
+    else if (item === Object(item)) Object.keys(item).forEach(
+      name => {
+        if (item[name]) push(name)
+      }
+    )
+
+    // otherwise, push any other values as a string
+    else if (item = item != null && String(item).trim()) item.split(/\s+/).forEach(
+      (name: string) => {
+        hash[name] = true
+      }
+    )
+  }
+}
+
 export function defineStyleVars(astroId: string, vars: Record<any, any>) {
   let output = '\n';
   for (const [key, value] of Object.entries(vars)) {

packages/astro/test/astro-class-list.test.js

@@ -0,0 +1,38 @@
+import { expect } from 'chai';
+import cheerio from 'cheerio';
+import { loadFixture } from './test-utils.js';
+
+let fixture;
+
+before(async () => {
+  fixture = await loadFixture({ projectRoot: './fixtures/astro-class-list/' });
+  await fixture.build();
+});
+
+describe('Class List', async () => {
+  it('Passes class:list attributes as expected to elements', async () => {
+    const html = await fixture.readFile('/index.html');
+    const $ = cheerio.load(html);
+
+    expect($('[class="test control"]')).to.have.lengthOf(1);
+    expect($('[class="test expression"]')).to.have.lengthOf(1);
+    expect($('[class="test true"]')).to.have.lengthOf(1);
+    expect($('[class="test truthy"]')).to.have.lengthOf(1);
+    expect($('[class="test set"]')).to.have.lengthOf(1);
+    expect($('[class="hello goodbye world friend"]')).to.have.lengthOf(1);
+
+    expect($('.false, .noshow1, .noshow2, .noshow3, .noshow4')).to.have.lengthOf(0);
+  });
+
+  it('Passes class:list attributes as expected to components', async () => {
+    const html = await fixture.readFile('/component/index.html');
+    const $ = cheerio.load(html);
+
+    expect($('[class="test control"]')).to.have.lengthOf(1);
+    expect($('[class="test expression"]')).to.have.lengthOf(1);
+    expect($('[class="test true"]')).to.have.lengthOf(1);
+    expect($('[class="test truthy"]')).to.have.lengthOf(1);
+    expect($('[class="test set"]')).to.have.lengthOf(1);
+    expect($('[class="hello goodbye world friend"]')).to.have.lengthOf(1);
+  });
+});

packages/astro/test/fixtures/astro-class-list/src/components/Span.astro

@@ -0,0 +1 @@
+<span {...Astro.props} />

packages/astro/test/fixtures/astro-class-list/src/pages/component.astro

@@ -0,0 +1,18 @@
+---
+import Component from '../components/Span.astro'
+---
+<Component class="test control" />
+
+<!-- @note: `class:list` will not be parsed if its value is not an expression -->
+<!-- <Component class:list="test" /> -->
+
+<Component class:list={'test expression'} />
+
+<Component class:list={[ 'array' ]} />
+
+<Component class:list={{ test: true, true: true, false: false }} />
+<Component class:list={{ test: 1, truthy: '0', noshow1: 0, noshow2: '', noshow3: null, noshow4: undefined }} />
+
+<Component class:list={new Set(['test', 'set'])} />
+
+<Component class:list={[ 'hello goodbye', { hello: true, world: true }, new Set([ 'hello', 'friend' ]) ]} />

packages/astro/test/fixtures/astro-class-list/src/pages/index.astro

@@ -0,0 +1,15 @@
+<span class="test control" />
+
+<!-- @note: `class:list` will not be parsed if its value is not an expression -->
+<!-- <span class:list="test" /> -->
+
+<span class:list={'test expression'} />
+
+<span class:list={[ 'array' ]} />
+
+<span class:list={{ test: true, true: true, false: false }} />
+<span class:list={{ test: 1, truthy: '0', noshow1: 0, noshow2: '', noshow3: null, noshow4: undefined }} />
+
+<span class:list={new Set(['test', 'set'])} />
+
+<span class:list={[ 'hello goodbye', { hello: true, world: true }, new Set([ 'hello', 'friend' ]) ]} />