https://developers.weixin.qq.com/miniprogram/dev/component/canvas.html

小程序插件中

wx.createSelectorQuery().select('#mycanvas').node(res => {

      console.log("node ",res)

      const canvas = res.node

      const context = canvas.getContext('2d')

    }).exec()

wx.createSelectorQuery().select('#mycanvas')能获取到,node()中res为null