index.js

const { spawn } = require('child_process')

// Defaults for image thumbnails
const imageOptions = {
  width: 100,
  height: 100,
  preserveAspectRatio: true
}

// Defaults for video thumbnails
const videoOptions = {
  width: 100,
  height: -1
}

const VIDEO_TYPE = 'video'
const IMAGE_TYPE = 'image'

/**
 * A pure function to generate thumbnail for an image or a video.
 * 
 * @param {string} source path to source image file
 * @param {string} destination path to destination folder
 * @param {object} options configuration options
 * @param {string} type video|image
 */
const thumbGenerator = (source, destination, options, type) =>
  new Promise((resolve, reject) => {
    if (!source || !destination) {
      reject(new Error('Source or destination path missing'))
    }

    // console.log(options)
    let convert
    if (type === IMAGE_TYPE) {
      convert = spawn(
        'convert', 
        [ source,
          '-thumbnail',
          `${options.width}x${options.height}${options.preserveAspectRatio ? '' : '!'}`,
          destination 
        ])
    } else {
      convert = spawn(
        'ffmpeg', 
        [ `-i ${source}`,
          `-vf "thumbnail,scale=${options.width}:${options.height}" -vframes 1`,
          '-nostdin -y',
          destination 
        ], { shell: true })
    }

    // convert.stdout.on('data', data => console.log(data))
    // convert.stderr.on('data', data => console.error(data))

    convert.on('exit', code => {
      if (code !== 0) {
        reject(new Error('Non-zero exit code'))
      }

      resolve()
    })
  })

const forImage = (source, destination, options) =>
  thumbGenerator(source, destination, Object.assign({}, imageOptions, options), IMAGE_TYPE)

const forVideo = (source, destination, options) =>
  thumbGenerator(source, destination, Object.assign({}, videoOptions, options), VIDEO_TYPE)

module.exports = {
  forImage,
  forVideo
}