feat: support multiple transform classes on single element

Add mergeStyles utility that concatenates transform arrays instead of
overwriting them. This enables combining multiple transforms like
'rotate-45 scale-110' on a single element.

Changes:
- Add mergeStyles() utility with smart array concatenation
- Update parseClassName() to use mergeStyles instead of Object.assign
- Add 9 unit tests for mergeStyles utility
- Add 10 integration tests for multiple transform combinations
- Export mergeStyles from main index for advanced usage

Author: mgcrea

src/index.ts

@@ -9,6 +9,7 @@ export { tw, twStyle } from "./stubs/tw";
 // Main parser functions
 export { parseClass, parseClassName } from "./parser";
 export { flattenColors } from "./utils/flattenColors";
+export { mergeStyles } from "./utils/mergeStyles";
 export { generateStyleKey } from "./utils/styleKey";
 
 // Re-export types

src/parser/index.ts

@@ -4,6 +4,7 @@
  */
 
 import type { StyleObject } from "../types";
+import { mergeStyles } from "../utils/mergeStyles";
 import { parseAspectRatio } from "./aspectRatio";
 import { parseBorder } from "./borders";
 import { parseColor } from "./colors";
@@ -36,7 +37,7 @@ export function parseClassName(className: string, customTheme?: CustomTheme): St
 
   for (const cls of classes) {
     const parsedStyle = parseClass(cls, customTheme);
-    Object.assign(style, parsedStyle);
+    mergeStyles(style, parsedStyle);
   }
 
   return style;

src/parser/transforms.test.ts

@@ -1,4 +1,5 @@
 import { describe, expect, it } from "vitest";
+import { parseClassName } from "./index";
 import { PERSPECTIVE_SCALE, ROTATE_MAP, SCALE_MAP, SKEW_MAP, parseTransform } from "./transforms";
 
 describe("SCALE_MAP", () => {
@@ -373,3 +374,85 @@ describe("parseTransform - custom spacing", () => {
     expect(parseTransform("perspective-500", customSpacing)).toEqual({ transform: [{ perspective: 500 }] });
   });
 });
+
+describe("parseClassName - multiple transforms", () => {
+  it("should combine rotate and scale transforms", () => {
+    const result = parseClassName("rotate-45 scale-110");
+    expect(result).toEqual({
+      transform: [{ rotate: "45deg" }, { scale: 1.1 }],
+    });
+  });
+
+  it("should combine scale and rotate with arbitrary values", () => {
+    const result = parseClassName("rotate-45 scale-[0.2]");
+    expect(result).toEqual({
+      transform: [{ rotate: "45deg" }, { scale: 0.2 }],
+    });
+  });
+
+  it("should combine multiple different transforms", () => {
+    const result = parseClassName("rotate-45 scale-110 translate-x-4");
+    expect(result).toEqual({
+      transform: [{ rotate: "45deg" }, { scale: 1.1 }, { translateX: 16 }],
+    });
+  });
+
+  it("should preserve order of transforms", () => {
+    // Order matters in React Native transforms!
+    const result1 = parseClassName("rotate-45 scale-110");
+    const result2 = parseClassName("scale-110 rotate-45");
+
+    expect(result1.transform).toEqual([{ rotate: "45deg" }, { scale: 1.1 }]);
+    expect(result2.transform).toEqual([{ scale: 1.1 }, { rotate: "45deg" }]);
+  });
+
+  it("should combine transforms with other properties", () => {
+    const result = parseClassName("m-4 rotate-45 scale-110 p-2");
+    expect(result).toEqual({
+      margin: 16,
+      padding: 8,
+      transform: [{ rotate: "45deg" }, { scale: 1.1 }],
+    });
+  });
+
+  it("should handle all transform types together", () => {
+    const result = parseClassName("perspective-500 rotate-45 scale-110 translate-x-4 skew-x-6");
+    expect(result).toEqual({
+      transform: [
+        { perspective: 500 },
+        { rotate: "45deg" },
+        { scale: 1.1 },
+        { translateX: 16 },
+        { skewX: "6deg" },
+      ],
+    });
+  });
+
+  it("should handle negative transforms", () => {
+    const result = parseClassName("-rotate-45 -translate-x-4");
+    expect(result).toEqual({
+      transform: [{ rotate: "-45deg" }, { translateX: -16 }],
+    });
+  });
+
+  it("should handle scale-x and scale-y together", () => {
+    const result = parseClassName("scale-x-50 scale-y-150");
+    expect(result).toEqual({
+      transform: [{ scaleX: 0.5 }, { scaleY: 1.5 }],
+    });
+  });
+
+  it("should handle single transform (backward compatibility)", () => {
+    const result = parseClassName("rotate-45");
+    expect(result).toEqual({
+      transform: [{ rotate: "45deg" }],
+    });
+  });
+
+  it("should handle arbitrary values combined", () => {
+    const result = parseClassName("rotate-[37deg] scale-[0.2] translate-x-[50px]");
+    expect(result).toEqual({
+      transform: [{ rotate: "37deg" }, { scale: 0.2 }, { translateX: 50 }],
+    });
+  });
+});

src/utils/mergeStyles.test.ts

@@ -0,0 +1,83 @@
+import { describe, expect, it } from "vitest";
+import { mergeStyles } from "./mergeStyles";
+
+describe("mergeStyles", () => {
+  describe("standard properties", () => {
+    it("should merge non-array properties like Object.assign", () => {
+      const target = { margin: 4 };
+      const source = { padding: 8 };
+      expect(mergeStyles(target, source)).toEqual({ margin: 4, padding: 8 });
+    });
+
+    it("should overwrite non-array properties", () => {
+      const target = { margin: 4 };
+      const source = { margin: 8 };
+      expect(mergeStyles(target, source)).toEqual({ margin: 8 });
+    });
+
+    it("should handle shadowOffset as standard property (object, not array)", () => {
+      const target = { shadowOffset: { width: 0, height: 1 } };
+      const source = { shadowOffset: { width: 0, height: 2 } };
+      expect(mergeStyles(target, source)).toEqual({
+        shadowOffset: { width: 0, height: 2 },
+      });
+    });
+  });
+
+  describe("transform array merging", () => {
+    it("should concatenate transform arrays", () => {
+      const target = { transform: [{ rotate: "45deg" }] };
+      const source = { transform: [{ scale: 1.1 }] };
+      expect(mergeStyles(target, source)).toEqual({
+        transform: [{ rotate: "45deg" }, { scale: 1.1 }],
+      });
+    });
+
+    it("should handle multiple transforms in source", () => {
+      const target = { transform: [{ rotate: "45deg" }] };
+      const source = { transform: [{ scale: 1.1 }, { translateX: 10 }] };
+      expect(mergeStyles(target, source)).toEqual({
+        transform: [{ rotate: "45deg" }, { scale: 1.1 }, { translateX: 10 }],
+      });
+    });
+
+    it("should assign transform when target has no transform", () => {
+      const target = { margin: 4 };
+      const source = { transform: [{ scale: 1.1 }] };
+      expect(mergeStyles(target, source)).toEqual({
+        margin: 4,
+        transform: [{ scale: 1.1 }],
+      });
+    });
+
+    it("should handle empty target transform array", () => {
+      const target = { transform: [] as Array<{ scale?: number }> };
+      const source = { transform: [{ scale: 1.1 }] };
+      expect(mergeStyles(target, source)).toEqual({
+        transform: [{ scale: 1.1 }],
+      });
+    });
+  });
+
+  describe("mixed properties", () => {
+    it("should handle mix of standard and transform properties", () => {
+      const target = { margin: 4, transform: [{ rotate: "45deg" }] };
+      const source = { padding: 8, transform: [{ scale: 1.1 }] };
+      expect(mergeStyles(target, source)).toEqual({
+        margin: 4,
+        padding: 8,
+        transform: [{ rotate: "45deg" }, { scale: 1.1 }],
+      });
+    });
+  });
+
+  describe("mutation behavior", () => {
+    it("should mutate and return target object", () => {
+      const target = { margin: 4 };
+      const source = { padding: 8 };
+      const result = mergeStyles(target, source);
+      expect(result).toBe(target);
+      expect(target).toEqual({ margin: 4, padding: 8 });
+    });
+  });
+});

src/utils/mergeStyles.ts

@@ -0,0 +1,56 @@
+/**
+ * Smart merge utility for StyleObject values
+ * Handles array properties (like transform) by concatenating instead of overwriting
+ */
+
+import type { StyleObject } from "../types/core";
+
+/**
+ * Properties that should be merged as arrays (concatenated) rather than overwritten
+ */
+const ARRAY_MERGE_PROPERTIES = new Set<string>(["transform"]);
+
+/**
+ * Merge two StyleObject instances, handling array properties specially
+ *
+ * @param target - The target object to merge into (mutated)
+ * @param source - The source object to merge from
+ * @returns The merged target object
+ *
+ * @example
+ * // Standard properties are overwritten (like Object.assign)
+ * mergeStyles({ margin: 4 }, { padding: 8 })
+ * // => { margin: 4, padding: 8 }
+ *
+ * @example
+ * // Array properties (transform) are concatenated
+ * mergeStyles(
+ *   { transform: [{ rotate: '45deg' }] },
+ *   { transform: [{ scale: 1.1 }] }
+ * )
+ * // => { transform: [{ rotate: '45deg' }, { scale: 1.1 }] }
+ */
+export function mergeStyles(target: StyleObject, source: StyleObject): StyleObject {
+  for (const key in source) {
+    if (Object.prototype.hasOwnProperty.call(source, key)) {
+      const sourceValue = source[key];
+
+      // Handle array merge properties (like transform)
+      if (ARRAY_MERGE_PROPERTIES.has(key) && Array.isArray(sourceValue)) {
+        const targetValue = target[key];
+        if (Array.isArray(targetValue)) {
+          // Concatenate arrays
+          (target as Record<string, unknown>)[key] = [...targetValue, ...sourceValue];
+        } else {
+          // No existing array, just assign
+          target[key] = sourceValue;
+        }
+      } else {
+        // Standard Object.assign behavior for non-array properties
+        target[key] = sourceValue;
+      }
+    }
+  }
+
+  return target;
+}