CanvasRenderingContext2D is not defined？

CanvasRenderingContext2D.prototype.clear = function() {
....
}

 时遇到该问题，但我看通过以下代码：
  			const query = wx.createSelectorQuery()
  			query.select('#myCanvas')
  				.fields({
  					node: true,
  					size: true
  				})
  				.exec((res) => {
  					const canvas = res[0].node
  					this.canvas = canvas;
  					const ctx = canvas.getContext('2d')
                            ...
                            }
 获取到的ctx 对象就是CanvasRenderingContext2D类型的？