https://api.weixin.qq.com/sns/oauth2/access_token?appid=APPID&secret=SECRET&code=CODE&grant_type=authorization_code
{"access_token":"93_R3z7PmuV-D5uURhxXKGav9xD0Cn8csRYO2WM2_8kkUZvOPxwsvq2FAKvHbf-97WARVG1H66YgleHPku40RKM-2A1xFv-D1MVpRSPkuN5x1Y","expires_in":7200,"refresh_token":"93_UEJyMzsiYwURcj9SVJfHC6YU9csFtokxKa9RWoaKliVmTeQWDVoQr3HOxxZ5rYgVgaBhbNRBR8Kk2K53f87pOxNOLvGWC9UL1plsCJthYt4","openid":"oPMcL0iFy6KsX4Bt4m_nr0xc3GJw","scope":"snsapi_base"}
你当前的授权作用域是 snsapi_base ，不会返回 unionid 。
当你的服务号绑定在开放平台账号下，且只有当scope为"snsapi_userinfo"时返回 unionid ，可参考文档：https://developers.weixin.qq.com/doc/service/OA_Web_Apps/Wechat_webpage_authorization.html