add ability to merge arrays of objects when using extend

__tests__/resolveConfig.test.js

@@ -810,7 +810,7 @@ test('theme values in the extend section can extend values that are depended on
   })
 })
 
-test('theme values in the extend section are not deeply merged', () => {
+test('theme values in the extend section are not deeply merged when they are simple arrays', () => {
   const userConfig = {
     theme: {
       extend: {
@@ -856,6 +856,76 @@ test('theme values in the extend section are not deeply merged', () => {
   })
 })
 
+test('theme values in the extend section are deeply merged, when they are arrays of objects', () => {
+  const userConfig = {
+    theme: {
+      extend: {
+        typography: {
+          ArrayArray: {
+            css: [{ a: { backgroundColor: 'red' } }, { a: { color: 'green' } }],
+          },
+          ObjectArray: {
+            css: { a: { backgroundColor: 'red' } },
+          },
+          ArrayObject: {
+            css: [{ a: { backgroundColor: 'red' } }, { a: { color: 'green' } }],
+          },
+        },
+      },
+    },
+  }
+
+  const defaultConfig = {
+    prefix: '-',
+    important: false,
+    separator: ':',
+    theme: {
+      typography: {
+        ArrayArray: {
+          css: [{ a: { underline: 'none' } }],
+        },
+        ObjectArray: {
+          css: [{ a: { underline: 'none' } }],
+        },
+        ArrayObject: {
+          css: { a: { underline: 'none' } },
+        },
+      },
+    },
+    variants: {},
+  }
+
+  const result = resolveConfig([userConfig, defaultConfig])
+
+  expect(result).toMatchObject({
+    prefix: '-',
+    important: false,
+    separator: ':',
+    theme: {
+      typography: {
+        ArrayArray: {
+          css: [
+            { a: { underline: 'none' } },
+            { a: { backgroundColor: 'red' } },
+            { a: { color: 'green' } },
+          ],
+        },
+        ObjectArray: {
+          css: [{ a: { underline: 'none' } }, { a: { backgroundColor: 'red' } }],
+        },
+        ArrayObject: {
+          css: [
+            { a: { underline: 'none' } },
+            { a: { backgroundColor: 'red' } },
+            { a: { color: 'green' } },
+          ],
+        },
+      },
+    },
+    variants: {},
+  })
+})
+
 test('the theme function can use a default value if the key is missing', () => {
   const userConfig = {
     theme: {

src/util/resolveConfig.js

@@ -7,6 +7,8 @@ import map from 'lodash/map'
 import get from 'lodash/get'
 import uniq from 'lodash/uniq'
 import toPath from 'lodash/toPath'
+import head from 'lodash/head'
+import isPlainObject from 'lodash/isPlainObject'
 import negateValue from './negateValue'
 import { corePluginList } from '../corePluginList'
 import configurePlugins from './configurePlugins'
@@ -67,8 +69,24 @@ function mergeThemes(themes) {
   }
 }
 
-function mergeExtensionCustomizer(_merged, value) {
-  if (Array.isArray(value)) return value
+function mergeExtensionCustomizer(merged, value) {
+  // When we have an array of objects, we do want to merge it
+  if (Array.isArray(merged) && isPlainObject(head(merged))) {
+    return merged.concat(value)
+  }
+
+  // When the incoming value is an array, and the existing config is an object, prepend the existing object
+  if (Array.isArray(value) && isPlainObject(head(value)) && isPlainObject(merged)) {
+    return [merged, ...value]
+  }
+
+  // Override arrays (for example for font-families, box-shadows, ...)
+  if (Array.isArray(value)) {
+    return value
+  }
+
+  // Execute default behaviour
+  return undefined
 }
 
 function mergeExtensions({ extend, ...theme }) {