fix(Field): improve cursor position handling for emoji input and formatting (#13711)

packages/vant/src/field/Field.tsx

@@ -329,8 +329,7 @@ export default defineComponent({
       // When the value length exceeds maxlength,
       // record the excess length for correcting the cursor position.
       // https://github.com/youzan/vant/issues/11289
-      const limitDiffLen =
-        getStringLength(originalValue) - getStringLength(value);
+      const limitDiffLen = originalValue.length - value.length;
 
       // https://github.com/youzan/vant/issues/13058
       if (props.type === 'number' || props.type === 'digit') {
@@ -368,8 +367,7 @@ export default defineComponent({
           const bcoVal = cutString(originalValue, selectionEnd!);
           // Record the length change of `bcoVal` after formatting,
           // which is used to correct the cursor position.
-          formatterDiffLen =
-            getStringLength(formatter(bcoVal)) - getStringLength(bcoVal);
+          formatterDiffLen = formatter(bcoVal).length - bcoVal.length;
         }
       }
 
@@ -380,7 +378,7 @@ export default defineComponent({
           inputRef.value.value = value;
 
           if (isDef(selectionStart) && isDef(selectionEnd)) {
-            const valueLen = getStringLength(value);
+            const valueLen = value.length;
 
             if (limitDiffLen) {
               selectionStart -= limitDiffLen;

packages/vant/src/field/test/index.spec.js

@@ -608,3 +608,39 @@ test("should not be set label's for attribute when using input slot", async () =
     wrapper.find('.van-field__label label').attributes('for'),
   ).toBeUndefined();
 });
+
+test('should update selection range correctly when inputting text into string with emoji', async () => {
+  const wrapper = mount(Field, {
+    props: {
+      maxlength: 2,
+      modelValue: '😀😀',
+    },
+  });
+
+  const input = wrapper.find('input');
+  await input.trigger('focus');
+
+  input.element.value = '😀😀😀';
+  input.element.selectionEnd = 6;
+  input.trigger('input');
+
+  expect(input.element.selectionEnd).toEqual(4);
+});
+
+test('should update selection range correctly when using formatter with emoji', async () => {
+  const wrapper = mount(Field, {
+    props: {
+      modelValue: '',
+      formatter: (val) => val.replace('1', '😀😀'),
+    },
+  });
+
+  const input = wrapper.find('input');
+  await input.trigger('focus');
+
+  input.element.value = '1';
+  input.element.selectionEnd = 1;
+  input.trigger('input');
+
+  expect(input.element.selectionEnd).toEqual(4);
+});