image optimisation via canvas api

optimiseImage.js

/**
  * @description promisified optimisation of image via canvas API
  *
  * @param {File} file - a file of an image which should be transformed
  * @param {integer} destHeight - a desired height of an output image Blob|DataURL
  * @param {integer} quality - a desired level of compression of an output image Blob|DataURL
  * @param {string} mimeType - mimeType of of an output image Blob|DataURL
  * @param {string} to - part of canvas conversion method name (can be 'DataURL' or 'Blob' as a `Canvas.toDataURL()` or `Canvas.toBlob()`)
  *
  * @return {Promise}
  */
  function optimiseImage(file, destHeight, mimeType, quality, to) {
    return new Promise( (resolve, reject) => {
      const reader = new FileReader()

      reader.onabort = () => reject('file reading was aborted')
      reader.onerror = () => reject('file reading has failed. try once again')

      reader.onload = () => {
        const result = reader.result,
              img = new Image()

        img.onload = () => {
          const resizeMultiplier = destHeight / img.height,
                canvas = document.createElement('canvas'),
                destWidth = Math.round(img.width * resizeMultiplier)

          canvas.width = destWidth
          canvas.height = destHeight
          canvas.getContext('2d').drawImage(img, 0, 0, destWidth, destHeight)

          if(to === 'DataURL') {
            // console.log('working on DataURL', canvas.toDataURL(mimeType, quality));
            resolve(canvas.toDataURL(mimeType, quality))
          } else if(to === 'Blob') {
            canvas.toBlob(blob => {
              resolve(blob)
            }, mimeType, quality)
          } else {
            reject( 'Wrong param to "value": allowed "DataURL" or "Blob"' )
          }
        } // img.onload end

        img.src = result
      } // reader.onload end

      reader.readAsDataURL(file)
    } )
  }