SOURCE

function draw(){
    const ctx = document.getElementById('canvas').getContext('2d');
    const img = new Image()
    img.onload = function(){
      ctx.drawImage(img,0,0); //在canvas的(0,0)处绘制图片
      ctx.beginPath()
      ctx.moveTo(30, 100)
      ctx.lineTo(100,80)
      ctx.lineTo(130, 15)
      ctx.lineTo(150,95)
      ctx.stroke()
    }
    img.src = 'https://mdn.mozillademos.org/files/5395/backdrop.png';
}

draw()

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