小程序中的typeof cb == “function”解说

getopenid: function (cb) {
    var that = this;
    if (this.globalData.openid) {
      typeof cb == "function" && cb(this.globalData.openid)
    } else {
      wx.login({
        success: res => {
          let code = res.code;
          wx.request({
            url: 'https://xxxx/api/WX/GetOpenId',
            method: "POST",
            data: {
              "WXOpendID": code,
              "WXAppID": "wx496dc0e3f80a0a6f",
            },
            success: function (response) {
              if (response.data != null && response.data != "err") {               
                wx.setStorageSync('openid', response.data)
                that.globalData.openid = response.data
                typeof cb == "function" && cb(that.globalData.openid)
              }
            },
            fail: function (error) {
              console.log('获取openid失败');
            }
          })
        }
      })
    }
  },