html

<canvas id="canvas"></canvas>

js

        var canvas = document.getElementById('canvas')
        var ctx = canvas.getContext('2d')
        var width = document.documentElement.clientWidth
        var height = document.documentElement.clientHeight
        canvas.setAttribute('width'
 
,width)
        canvas.setAttribute('height',height)
        var centerX = width / 2
        var centerY = height / 2
        ctx.beginPath()
        // ctx.arc(100, 100, 50, 0, 360, false)
        var lingrad = ctx.createRadialGradient(centerX, centerY, 0, centerX, centerY, centerX)  //引數分別為 內圓x座標,y座標,半徑,外圓x座標,y座標,半徑
 

        lingrad.addColorStop(0, '#0e1c27')      //addColorStop()設定漸變範圍及顏色
        lingrad.addColorStop(0.5  , '#02cf99')
        lingrad.addColorStop(1, '#23253c')

        // console.log(width,height,centerX,centerY)
        ctx.fillStyle = lingrad  
        ctx.fillRect(0, 0, width, height)
        // ctx.stroke()
 

        ctx.closePath()

效果: