The majority of this site draws everything with triangles. This is arguably the normal thing that 99% of WebGL programs do. But, for the sake of completeness let's go over a few other cases.

As mentioned in the first article WebGL draws points, lines, and triangles. It does this when we call gl.drawArrays or gl.drawElements. We provide a vertex shader that outputs clip space coordinates and then, based on the first argument to gl.drawArrays or gl.drawElements WebGL will draw points, lines, or triangles.

The valid values for the first argument to gl.drawArrays and gl.drawElements are

• POINTS

  For each clip space vertex output by the vertex shader draw a square centered over that point. The size of the square is specified by setting a special variable gl_PointSize inside the vertex shader to the size we want for this square in pixels.

  Note: The maximum (and minimum) size that square can be is implementation dependent which you can query with

    const [minSize, maxSize] = gl.getParameter(gl.ALIASED_POINT_SIZE_RANGE);
  

  Also see another issue here.

• LINES

  For each 2 clip space vertices output by the vertex shader draw a line connecting the 2 points. If we had points A,B,C,D,E,F then we'd get 3 lines.

  

  The spec says we can set the thickness of this line by calling gl.lineWidth and specifying a width in pixels. In reality though the maximum width is implementation dependent and for the majority of implementations the maximum width is 1.

    const [minSize, maxSize] = gl.getParameter(gl.ALIASED_LINE_WIDTH_RANGE);
  

  This is mostly because values > 1 have been deprecated in core Desktop OpenGL.

• LINE_STRIP

  For each clip space vertex output by the vertex shader draw a line from the previous point output by the vertex shader.

  So, if you output clip space vertices A,B,C,D,E,F you'll get 5 lines.

  

• LINE_LOOP

  This is the same as LINE_STRIP example one more line is drawn from the last point to the first point.

  

• TRIANGLES

  For every 3 clip space vertices output by the vertex shader draw a triangle from the 3 points. This is the most used mode.

  

• TRIANGLE_STRIP

  For each clip space vertex output by the vertex shader draw a triangle from the last 3 vertices. In other words If you output 6 points A,B,C,D,E,F then 4 triangles will be drawn. A,B,C then B,C,D then C,D,E then D,E,F

  

• TRIANGLE_FAN

  For each clip space vertex output by the vertex shader draw a triangle from the first vertex and the last 2 vertices. In other words if you output 6 points A,B,C,D,E,F then 4 triangles will be drawn. A,B,C then A,C,D then A,D,E and finally A,E,F

  

I'm sure some others will disagree but in my experience TRIANGLE_FAN and TRIANGLE_STRIP are best avoided. They fit only a few exceptional cases and the extra code for handling those cases is not worth just doing everything in triangles in the first place. In particular maybe you have tools to build normals or generate texture coordinates or do any other number of things with vertex data. By sticking to just TRIANGLES your functions will just work. As soon as you start adding in TRIANGLE_FAN and TRIANGLE_STRIP you need more functions to handle more cases. You're free to disagree and do whatever you want. I'm just saying that's my experience and the experience of a few AAA game devs I've asked.

Similarly LINE_LOOP and LINE_STRIP are not so useful and have similar issues. Like TRIANGLE_FAN and TRIANGLE_STRIP the situations to use them are rare. For example you might think you want to draw 4 connected lines each made from 4 points.

If you use LINE_STRIP you'd need to make 4 calls to gl.drawArrays and more calls to setup the attributes for each line whereas if you just use LINES then you can insert all the points needed to draw all 4 sets of lines with a single call to gl.drawArrays. That will be much much faster.

Further, LINES can be great to use for debugging or simple effects but given their 1 pixel width limit on most platforms it's often the wrong solution. If you want to draw a grid for a graph or show the outlines of polygons in a 3d modeling program using LINES might be great but if you want to draw structured graphics like SVG or Adobe Illustrator then it won't work and you have to render your lines some other way, usually from triangles.