const fontHeight = 12;

if (width && height) {
  // canvas setup removed for brevity
  hiddenContext.drawImage(video, 0, 0, width, height);

  outputContext.textBaseline = 'top';
  outputContext.font = `${fontHeight}px Consolas`;

  const text = outputContext.measureText('@');
  const fontWidth = parseInt(text.width);

  // more processing
}