HarmonyOS 如何获取字符串的最后一个Unicode字符

@Entry
@Component
struct Index {
  build() {
    RelativeContainer() {
      Button('获取最后一个unicode').onClick(() => {
        const str: string = '\ud83d\udcda书上说，夏至是——哈哈\ud83c\udf49';
        const lastCharCode = str.charCodeAt(str.length - 1);
        const secondLastCharCode = str.charCodeAt(str.length - 2);
        const unicode = String.fromCharCode(secondLastCharCode, lastCharCode);
        console.log(unicode);
      })
    }
    .height('100%')
    .width('100%')
  }
}