fix: [el-checkbox-group]: default value adjusted to empty array (#4316)

fix #4300

docs/en-US/component/checkbox.md

@@ -106,7 +106,7 @@ checkbox/with-border
 
 | Attribute             | Description                                       | Type    | Accepted Values        | Default |
 | --------------------- | ------------------------------------------------- | ------- | ---------------------- | ------- |
-| model-value / v-model | binding value                                     | array   | —                      | —       |
+| model-value / v-model | binding value                                     | array   | —                      | []      |
 | size                  | size of checkbox                                  | string  | large / default /small | —       |
 | disabled              | whether the nesting checkboxes are disabled       | boolean | —                      | false   |
 | min                   | minimum number of checkbox checked                | number  | —                      | —       |

packages/components/checkbox/__tests__/checkbox.spec.ts

@@ -95,6 +95,24 @@ describe('Checkbox', () => {
     expect(vm.checkList).toContain('b')
   })
 
+  test('checkbox group without modelValue', async () => {
+    const wrapper = _mount(
+      `
+      <el-checkbox-group v-model="checkList">
+        <el-checkbox label="a" ref="a"></el-checkbox>
+        <el-checkbox label="b" ref="b"></el-checkbox>
+        <el-checkbox label="c" ref="c"></el-checkbox>
+        <el-checkbox label="d" ref="d"></el-checkbox>
+      </el-checkbox-group>
+      `,
+      () => ({ checkList: undefined })
+    )
+    const vm = wrapper.vm
+    await wrapper.findComponent({ ref: 'a' }).trigger('click')
+    expect(vm.checkList.length).toBe(1)
+    expect(vm.checkList).toContain('a')
+  })
+
   test('checkbox group change', async () => {
     const wrapper = _mount(
       `

packages/components/checkbox/src/checkbox-group.vue

@@ -26,8 +26,8 @@ export default defineComponent({
 
   props: {
     modelValue: {
-      type: [Object, Boolean, Array],
-      default: () => undefined,
+      type: Array,
+      default: () => [],
     },
     disabled: Boolean,
     min: {