index.js

/**
 * Expose `thumb()`.
 */

module.exports = thumb;

/**
 * Scale `img` to fit within `width` / `height`
 * and and invoke `fn(err, img)`.
 *
 * @param {String|Image} img or data uri
 * @param {Number} width
 * @param {Number} height
 * @param {Function} fn
 * @api public
 */

function thumb(img, width, height, fn, quality) {
  var canvas = document.createElement('canvas');
  var ctx = canvas.getContext('2d');

  if ('string' == typeof img) {
    fromURI(img, resize);
  } else {
    resize(null, img);
  }

  function resize(err, img) {
    if (err) return fn(err);

    var ratio = img.width / width > img.height / height
      ? img.width / width
      : img.height / height;

    if (ratio > 1) {
      width = Math.ceil(img.width / ratio);
      height = Math.ceil(img.height / ratio);
    } else {
      width = img.width;
      height = img.height;
    }

    canvas.width = width;
    canvas.height = height;
    ctx.drawImage(img, 0, 0, width, height);

    fromURI(canvas.toDataURL('image/jpeg', quality || .9), fn);
  }
}

/**
 * Return `Image` from data uri `str`
 * and invoke `fn(err, img)`.
 *
 * @param {String} str
 * @param {Function} fn
 * @api private
 */

function fromURI(str, fn) {
  var img = new Image
  img.onerror = fn;
  img.onload = function(e){ fn(null, img, str) };
  img.src = str;
}