add the ability to deep merge extended configs

__tests__/resolveConfig.test.js

@@ -229,6 +229,55 @@ test('theme key is merged instead of replaced', () => {
   })
 })
 
+test('theme key is deeply merged instead of replaced', () => {
+  const userConfig = {
+    theme: {
+      extend: {
+        colors: {
+          grey: {
+            darker: '#606f7b',
+            dark: '#8795a1',
+          },
+        },
+      },
+    },
+  }
+
+  const defaultConfig = {
+    prefix: '-',
+    important: false,
+    separator: ':',
+    theme: {
+      colors: {
+        grey: {
+          grey: '#b8c2cc',
+          light: '#dae1e7',
+          lighter: '#f1f5f8',
+        },
+      },
+    },
+  }
+
+  const result = resolveConfig([userConfig, defaultConfig])
+
+  expect(result).toMatchObject({
+    prefix: '-',
+    important: false,
+    separator: ':',
+    theme: {
+      colors: {
+        grey: {
+          darker: '#606f7b',
+          dark: '#8795a1',
+          grey: '#b8c2cc',
+          light: '#dae1e7',
+          lighter: '#f1f5f8',
+        },
+      },
+    },
+  })
+})
+
 test('variants key is merged instead of replaced', () => {
   const userConfig = {
     variants: {
@@ -1779,6 +1828,70 @@ test('variants can be extended', () => {
   })
 })
 
+test('extensions are applied in the right order', () => {
+  const userConfig = {
+    theme: {
+      extend: {
+        colors: {
+          grey: {
+            light: '#eee',
+          },
+        },
+      },
+    },
+  }
+
+  const otherConfig = {
+    theme: {
+      extend: {
+        colors: {
+          grey: {
+            light: '#ddd',
+            darker: '#111',
+          },
+        },
+      },
+    },
+  }
+
+  const anotherConfig = {
+    theme: {
+      extend: {
+        colors: {
+          grey: {
+            darker: '#222',
+          },
+        },
+      },
+    },
+  }
+
+  const defaultConfig = {
+    theme: {
+      colors: {
+        grey: {
+          light: '#ccc',
+          dark: '#333',
+        },
+      },
+    },
+  }
+
+  const result = resolveConfig([userConfig, otherConfig, anotherConfig, defaultConfig])
+
+  expect(result).toMatchObject({
+    theme: {
+      colors: {
+        grey: {
+          light: '#eee',
+          dark: '#333',
+          darker: '#111',
+        },
+      },
+    },
+  })
+})
+
 test('variant sort order can be customized', () => {
   const userConfig = {
     variantOrder: [

src/util/resolveConfig.js

@@ -67,20 +67,23 @@ function mergeThemes(themes) {
   }
 }
 
+function mergeExtensionCustomizer(_merged, value) {
+  if (Array.isArray(value)) return value
+}
+
 function mergeExtensions({ extend, ...theme }) {
   return mergeWith(theme, extend, (themeValue, extensions) => {
     // The `extend` property is an array, so we need to check if it contains any functions
     if (!isFunction(themeValue) && !some(extensions, isFunction)) {
-      return {
-        ...themeValue,
-        ...Object.assign({}, ...extensions),
-      }
+      return mergeWith({}, themeValue, ...extensions, mergeExtensionCustomizer)
     }
 
-    return (resolveThemePath, utils) => ({
-      ...value(themeValue, resolveThemePath, utils),
-      ...Object.assign({}, ...extensions.map((e) => value(e, resolveThemePath, utils))),
-    })
+    return (resolveThemePath, utils) =>
+      mergeWith(
+        {},
+        ...[themeValue, ...extensions].map((e) => value(e, resolveThemePath, utils)),
+        mergeExtensionCustomizer
+      )
   })
 }