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HTML

              
                <div id="contents"></div>
              
            
!

CSS

              
                body {
  margin: 0;
  padding: 0;
}

#contents {
  width: 100vw;
  height: 100vh;
}
              
            
!

JS

              
                // Canvas追加
const target = document.getElementById('contents')      // 追加先
const CreateCanvas = document.createElement('canvas')   // canvas element作成
CreateCanvas.setAttribute('id', 'canvas')               // canvas に id 付与
target.appendChild(CreateCanvas)                        // 追加

function draw() {
  // 追加先のサイズ取得
  let Cwidth = target.clientWidth
  let Cheight = target.clientHeight

  // Canvas サイズ設定
  CreateCanvas.width = Cwidth
  CreateCanvas.height = Cheight               

  // Canvas・コンテキスト取得
  const canvas = document.getElementById('canvas')
  const ctx = canvas.getContext('2d')

  // Canvasの中央取得
  let CenterW = Cwidth/2
  let CenterH = Cheight/2
  
  // 描画リセット
  ctx.clearRect(0, 0, Cwidth, Cheight) 

  // 円描画
  ctx.fillStyle = 'rgba(42, 54, 59, 1)'
  ctx.strokeStyle = 'rgba(232, 74, 95, 1)'
  
  ctx.beginPath()
  ctx.arc(CenterW - 10, CenterH - 10, 120, 0, Math.PI * 2, true)
  ctx.fill()
  
  ctx.beginPath()
  ctx.arc(CenterW + 10, CenterH + 10, 120, 0, Math.PI * 2, true)
  ctx.stroke()
  
  // ランダムで衛星生成
  ctx.beginPath()
  ctx.arc(CenterW + Math.floor(Math.random() * 30) - 150, CenterH + Math.floor(Math.random() * 30) - 150, Math.floor(Math.random() * 50) + 10, 0, Math.PI * 2, true)
  ctx.fill()
  
  ctx.beginPath()
  ctx.arc(CenterW + Math.floor(Math.random() * 30) - 150, CenterH + Math.floor(Math.random() * 30) - 150, Math.floor(Math.random() * 50) + 10, 0, Math.PI * 2, true)
  ctx.stroke()
  
  ctx.beginPath()
  ctx.arc(CenterW + Math.floor(Math.random() * 30) + 120, CenterH + Math.floor(Math.random() * 30) + 120, Math.floor(Math.random() * 50) + 10, 0, Math.PI * 2, true)
  ctx.stroke()
  
  // テキスト描画
  ctx.fillStyle = 'rgba(232, 74, 95, 1)'
  ctx.strokeStyle = 'rgba(255,255,255, 1)'
  ctx.font = '42px serif'
  
  // テキストサイズ取得
  let textSize = ctx.measureText('Text Test')  
  
  // テキスト塗りつぶし 
  ctx.fillText('Test Text', CenterW - (textSize.width / 2) -10, CenterH - 10)
  
  // テキストアウトライン  
  ctx.strokeText('Test Text', CenterW - (textSize.width / 2) + 10, CenterH + 10)
}

// 画面リサイズ時再描画
window.onresize = draw

// 初回描画
draw()
              
            
!
999px

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