Rectangle rotating at the origin.

Rectangle.js

class Rectangle {
  constructor(gl, color, width, height) {
    this.gl       = gl;
    this.color    = color;
    this.width    = width;
    this.height   = height;

    this.rotation = gl.mat2d.create();
  }

  /**
   * Update the Rectangle's transformation matrix before a render.
   */
  update(elapsedMs) {
    // Rotate PI per second.
    this.gl.mat2d.rotate(this.rotation, this.rotation,
      Math.PI * elapsedMs / 1000);
  }

  /**
   * Get the Rectangle's transformation matrix.
   */
  getTransformation() {
    return this.rotation;
  }
}