When does a base64 encoded png image not end with '='

Question

I have been working a lot lately with image manipulation in node using the sharp library and having my service return a base64 encoding of the new image. My question seems simple: does a base64 encoded image always end with

==

? If not, what are the differences when dealing with jpeg/png? My understanding is that encoding should be the same regardless of the format, but this makes me think otherwise. During my testing, I decided to have a unit test to verify if the body of my response(the base64 image) ends with

=

.
What I have found is that the test will pass if I do not crop the image by using

sharp.max()

to resize but keep the ratio. If I leave this off and resize to whatever height/width params I like, the test will fail. It is also worth noting that this behavior is limited to png, not jpeg.

Here is my resizing function:

const sharp = require('sharp');

const request = require('request');

const bufferRequest = request.defaults({ encoding: null });



function imgFormatter(url, args) {

    return new Promise((resolve, reject) => {

      bufferRequest.get(url, args, function (err, res, body) {

        let imgFormat = url.includes('png') ? 'png' : 'jpeg';

        let resized = (args.crop)

        ? sharp(body).resize(args.width, args.height)

        : sharp(body).resize(args.width, args.height).max()

        resized.toFormat(imgFormat)

        .toBuffer()

        .then((output) => {

          let newImage = "data:" + res.headers["content-type"] + ";base64," + new Buffer(output).toString('base64');

          console.log(newImage);

          resolve(newImage);

        })

        .catch((error) => {

          reject(error);

        })

      });

    })

}

Here is the end of the file looks like if cropped:

data:image/jpeg;base64,/9j/.....RgX7p57pbYTvawQC580N4QcJtygGGgG/SLplthgBV2KCfCgIa3wWTVA3HySPmgP//Z

Here is the image when NOT cropped:

data:image/jpeg;base64,/9j/.....4qUhXQD5S5HzQhAM8KRz80IQH//2Q==

Apologies for the snippet of examples, my base64 is so large I cannot include here. I am using https://codebeautify.org/base64-to-image-converter to test if the image is good, and I can see both generate images as expected in the correct sizing. Why does the cropped image encode to base64 not ending with

==

?
In case it is useful, here is the sharp library I am using: http://sharp.dimens.io/en/stable/api-resize/#max

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Answer 1

The = at the end isn't a terminator, it's padding:

https://en.wikipedia.org/wiki/Base64#Output_padding

So whether you see it or not depends on the size in bytes of the object you are encoding.

When does a base64 encoded png image not end with '='

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