Clearing by clicking

<p>
  A very simple WebGL program that still shows some color and user interaction.
</p>
<p>
  You can repeatedly click the empty canvas or the button below to change color.
</p>
<canvas id="canvas-view"
  >Your browser does not seem to support HTML5 canvas.</canvas
>
<button id="color-switcher">Press here to switch color</button>

body {
  text-align: center;
}
canvas {
  display: block;
  width: 280px;
  height: 210px;
  margin: auto;
  padding: 0;
  border: none;
  background-color: black;
}
button {
  display: inline-block;
  font-size: inherit;
  margin: auto;
  padding: 0.6em;
}

window.addEventListener(
  "load",
  function setupWebGL(evt) {
    "use strict";

    // Cleaning after ourselves. The event handler removes
    // itself, because it only needs to run once.
    window.removeEventListener(evt.type, setupWebGL, false);

    // 给 canvas 和 button 添加相同的时间处理器
    var canvas = document.querySelector("#canvas-view");
    var button = document.querySelector("#color-switcher");
    canvas.addEventListener("click", switchColor, false);
    button.addEventListener("click", switchColor, false);

    // 存放 WebGLRenderingContext 的变量
    var gl;

    // 点击事件处理器
    function switchColor() {
      // Referring to the externally defined gl variable.
      // If undefined, try to obtain the WebGLRenderingContext.
      // If failed, alert user of failure.
      // Otherwise, initialize the drawing buffer (the viewport).
      if (!gl) {
        gl =
          canvas.getContext("webgl") || canvas.getContext("experimental-webgl");
        if (!gl) {
          alert(
            "Failed to get WebGL context.\n" +
              "Your browser or device may not support WebGL.",
          );
          return;
        }
        gl.viewport(0, 0, gl.drawingBufferWidth, gl.drawingBufferHeight);
      }
      // 使用辅助函数获取一种随机色
      var color = getRandomColor();
      // 用随机色设置底色
      gl.clearColor(color[0], color[1], color[2], 1.0);
      // Clear the context with the newly set color. This is
      // the function call that actually does the drawing.
      gl.clear(gl.COLOR_BUFFER_BIT);
    }

    // 随机颜色辅助函数
    function getRandomColor() {
      return [Math.random(), Math.random(), Math.random()];
    }
  },
  false,
);