微信小程序获取van-field值

微信小程序使用vant的van-field组件，在js中怎么才能获取到他们的值呀？


<view style="margin-top: auto;">
    <van-cell-group>
      <van-field
        model:value="{{username}}"
        label="账号："
        placeholder="请输入账号"
        size="large"
        clearable
        required
      />

      <van-field
        model:value="{{password}}"
        type="password"
        label="密码："
        placeholder="请输入密码"
        size="large"
        clearable
        required
      />
    van-cell-group>
  view>

  <view class="public_button">
    <van-button type="primary" block custom-style="border-radius: 50px; width: 90vw" bindtap="index" bind:confirm="confirm">登录van-button>
  view>