typescript如何使用watch?

如何将下面的js版的watch，改成ts版：

watch: {
    checkVal: {
      handler: function (val) {
        if (val) {
          this.checkList.forEach((item) => {
            this.checkVal.forEach((i) => {
              if (i == item[this.idKey]) {
                this.checkRightList.push(item);
              }
            });
          });
          const res = new Map();
          this.checkRightList = this.checkRightList.filter(
            (a) => !res.has(a[this.idKey]) && res.set(a[this.idKey], 1)
          );
        }
      },
      deep: true,
    },

    showList: {
      handler: function (val) {
        if (val) {
          this.$emit('change', val);
        }
      },
      deep: true,
    },
  },