Question about simple triangle rendering about twgl.js

[greggman]

Here's working code:

import * as twgl from 'https://twgljs.org/dist/5.x/twgl-full.module.js';

function renderTriangle(el) {
  const vs = `
attribute vec4 position;

void main() {
  gl_Position = position;
}
`;

  const fs = `
precision mediump float;

void main() {
  // gl_FragColor is a special variable a fragment shader
  // is responsible for setting
  gl_FragColor = vec4(1, 0, 0.5, 1); // return redish-purple
}
`;

  const gl = twgl.getWebGLContext(el);
  const programInfo = twgl.createProgramInfo(gl, [vs, fs]);

  const arrays = {
    position: {
      numComponents: 2,
      data: [
        0, 0,
        0, 0.5,
        0.7, 0,
     ],
    }
  };

  gl.clearColor(0, 0, 0, 0);
  gl.clear(gl.COLOR_BUFFER_BIT);

  const bufferInfo = twgl.createBufferInfoFromArrays(gl, arrays);
  // const positionAttributeLocation = gl.getAttribLocation(programInfo.program, "position");

  twgl.resizeCanvasToDisplaySize(gl.canvas);
  gl.viewport(0, 0, gl.canvas.width, gl.canvas.height);

  gl.useProgram(programInfo.program);
  // gl.enableVertexAttribArray(positionAttributeLocation);
  twgl.setBuffersAndAttributes(gl, programInfo, bufferInfo);
  // gl.vertexAttribPointer(
  //   positionAttributeLocation, 2, gl.FLOAT, false, 0, 0);
  twgl.drawBufferInfo(gl, bufferInfo, gl.TRIANGLES, 3, 0);
  // gl.drawArrays(gl.TRIANGLES, 0, 3);
}
renderTriangle(document.getElementById('c2'));

Live: https://jsgist.org/?src=4f95e62f8d18a568856dbc45462675ce

All I had to do was change the this 6

  twgl.drawBufferInfo(gl, bufferInfo, gl.TRIANGLES, 6, 0);

to a 3

  twgl.drawBufferInfo(gl, bufferInfo, gl.TRIANGLES, 3, 0);

normally I'd leave it off

  twgl.drawBufferInfo(gl, bufferInfo);

gl.TRIANGLES is the default and if you don't pass a count it will take it from bufferInfo.numElements wi