code: function extractChannel (channel) {
    if (channel === 'red') {
      channel = 0;
    } else if (channel === 'green') {
      channel = 1;
    } else if (channel === 'blue') {
      channel = 2;
    }
    if (is.integer(channel) && is.inRange(channel, 0, 4)) {
      this.options.extractChannel = channel;
    } else {
      throw new Error('Cannot extract invalid channel ' + channel);
    }
    return this;
  }
docs: Extract a single channel from a multi-channel image.
  
  @example
  sharp(input)
  .extractChannel('green')
  .toFile('input_green.jpg', function(err, info) {
  // info.channels === 1
  // input_green.jpg contains the green channel of the input image
  });
  
  @param {Number|String} channel - zero-indexed band number to extract, or `red`, `green` or `blue` as alternative to `0`, `1` or `2` respectively.
  @returns {Sharp}
  @throws {Error} Invalid channel