Add composeStyles function

.changeset/small-guests-film.md

@@ -0,0 +1,5 @@
+---
+'@vanilla-extract/css': minor
+---
+
+Add `composeStyles` function

README.md

@@ -94,6 +94,7 @@ document.write(`
   - [globalFontFace](#globalfontface)
   - [keyframes](#keyframes)
   - [globalKeyframes](#globalkeyframes)
+  - [composeStyles](#composestyles)
 - [Dynamic API](#dynamic-api)
   - [createInlineTheme](#createinlinetheme)
   - [setElementTheme](#setelementtheme)
@@ -580,6 +581,28 @@ export const animated = style({
 });
 ```
 
+### composeStyles
+
+Combines mutliple styles into a single class string, while also deduplicating and removing unnecessary spaces.
+
+```ts
+import { style, composeStyles } from '@vanilla-extract/css';
+
+const base = style({
+  padding: 12
+});
+
+export const blue = composeStyles(base, style({
+  background: 'blue'
+}));
+
+export const green = composeStyles(base, style({
+  background: 'green'
+}));
+```
+
+> 💡 Styles can also be provided in shallow and deeply nested arrays. Think of it as a static version of [classnames.](https://github.com/JedWatson/classnames)
+
 ## Dynamic API
 
 We also provide a lightweight standalone package to support dynamic runtime theming.

packages/css/src/composeStyles.test.ts

@@ -0,0 +1,25 @@
+import { composeStyles } from './composeStyles';
+
+describe('composeStyles', () => {
+  it.each([
+    { args: ['1'], output: '1' },
+    { args: ['1 1'], output: '1' },
+    { args: ['1 2 3'], output: '1 2 3' },
+    { args: ['1', '2', '3'], output: '1 2 3' },
+    { args: [['1', '2'], '3'], output: '1 2 3' },
+    { args: [['1', ['2', '3']], '4'], output: '1 2 3 4' },
+    {
+      args: [['1', ['2', ['3', '4']]], ['5', '6'], '7'],
+      output: '1 2 3 4 5 6 7',
+    },
+    { args: ['1', '1', '1'], output: '1' },
+    {
+      args: ['1', ['1', '2'], ['1', '2', '3'], ['1', '2', '3', '4']],
+      output: '1 2 3 4',
+    },
+    { args: ['1 2 3', '2 3 4', '1 5'], output: '1 2 3 4 5' },
+    { args: [' 1  2  3  2 ', ' 2  3  4 2 ', ' 1  5  1 '], output: '1 2 3 4 5' },
+  ])('composeStyles', ({ args, output }) => {
+    expect(composeStyles(...args)).toBe(output);
+  });
+});

packages/css/src/composeStyles.ts

@@ -0,0 +1,30 @@
+type ClassNames = string | Array<ClassNames>;
+
+function composeStylesIntoSet(
+  set: Set<string>,
+  ...classNames: Array<ClassNames>
+) {
+  for (const className of classNames) {
+    if (className.length === 0) {
+      continue;
+    }
+
+    if (typeof className === 'string') {
+      if (className.includes(' ')) {
+        composeStylesIntoSet(set, ...className.trim().split(' '));
+      } else {
+        set.add(className);
+      }
+    } else if (Array.isArray(className)) {
+      composeStylesIntoSet(set, ...className);
+    }
+  }
+}
+
+export function composeStyles(...classNames: Array<ClassNames>) {
+  const set: Set<string> = new Set();
+
+  composeStylesIntoSet(set, ...classNames);
+
+  return Array.from(set).join(' ');
+}

packages/css/src/index.ts

@@ -5,3 +5,4 @@ export * from './identifier';
 export * from './theme';
 export * from './style';
 export * from './vars';
+export { composeStyles } from './composeStyles';