how can component support canvas.toDataUrl()

[jiangheng90]

I meet new issues for now，I have set context config below

let canvas = document.getElementById('#canvas')
canvas.getContext('webgl', {preserveDrawingBuffer: true})
console.log(canvas.toDataURL('image/jpeg',1.0))

it only appear blank on my base64url ， I‘m not quit familiar with webgl how can I get screen image in my unity canvase， thx