Base64

Canvas

Load the image into an Image-Object, paint it to a canvas and convert the canvas back to a dataURL.

FileReader

Load the image as blob via XMLHttpRequest and use the FileReader API to convert it to a data URL.

• lacks in browser support
• has better compression
• works for other file types as well.

References

[1] Coder_sLaY@StackOverflow, How to convert image into base64 string using javascript