How to save a base64-encoded image to disk ?

Once we create the buffer with the proper encoding, we just need to write the buffer to the file.

var base64Data = req.rawBody.replace(/^data:image\/png;base64,/, "");
 
require("fs").writeFile("out.png", base64Data, 'base64', function(err) {
console.log(err);
});

new Buffer(…, ‘base64’) converts the input string to a Buffer, which is just an array of bytes, by interpreting the input as a base64 encoded string and you can just write that byte array to the file.

This code will be helpful for any base64 image format and save it in the proper format in the database:

    // Save base64 image to disk
    try
    {
        // Decoding base-64 image
        // Source: http://stackoverflow.com/questions/20267939/nodejs-write-base64-image-file
        function decodeBase64Image(dataString)
        {
            var matches = dataString.match(/^data:([A-Za-z-+\/]+);base64,(.+)$/);
            var response = {};
 
        if (matches.length !== 3)
        {
            return new Error('Invalid input string');
        }
 
        response.type = matches[1];
        response.data = new Buffer(matches[2], 'base64');
 
        return response;
    }
 
    // Regular expression for image type:
    // This regular image extracts the "jpeg" from "image/jpeg"
    var imageTypeRegularExpression = /\/(.*?)$/;
 
    // Generate random string
    var crypto = require('crypto');
    var seed = crypto.randomBytes(20);
    var uniqueSHA1String = crypto
                             .createHash('sha1')
                             .update(seed)
                             .digest('hex');
 
    var base64Data =     data:image/jpeg;base64,/9j/4AAQSkZJRgABAQEAZABkAAD/4Q3zaHR0cDovL25zLmFkb2JlLmN...';
 
    var imageBuffer = decodeBase64Image(base64Data);
    var userUploadedFeedMessagesLocation = '../img/upload/feed/';
 
    var uniqueRandomImageName = 'image-' + uniqueSHA1String;
    // This variable is actually an array which has 5 values,
    // The [1] value is the real image extension
    var imageTypeDetected = imageBuffer
                                       .type
                                    .match(imageTypeRegularExpression);
 
    var userUploadedImagePath = userUploadedFeedMessagesLocation +
                                               uniqueRandomImageName +
                                                '.' +
                                                imageTypeDetected[1];
 
    // Save decoded binary image to disk
    try
    {
    require('fs').writeFile(userUploadedImagePath, imageBuffer.data,
                                 function()
                                {
                                 console.log('DEBUG - feed:message: Saved to disk image attached by user:',      userUploadedImagePath);
                                 });
    }
    catch(error)
    {
    console.log('ERROR:', error);
    }
 
}
catch(error)
{
    console.log('ERROR:', error);
}

This code will Convert file with base64 string to png image.

In this four variants are used.

var {promisify} = require('util');
var fs = require("fs");
var readFile = promisify(fs.readFile)
var writeFile = promisify(fs.writeFile)
async function run () {
 
    // variant 1
    var d = await readFile('./1.txt', 'utf8')
    await writeFile("./1.png", d, 'base64')
 
    // variant 2
    var d = await readFile('./2.txt', 'utf8')
    var dd = new Buffer(d, 'base64')
    await writeFile("./2.png", dd)
 
    // variant 3
    var d = await readFile('./3.txt')
    await writeFile("./3.png", d.toString('utf8'), 'base64')
 
    // variant 4
    var d = await readFile('./4.txt')
    var dd = new Buffer(d.toString('utf8'), 'base64')
    await writeFile("./4.png", dd)
}
run();

Once you create the buffer with the proper encoding, you just need to write the buffer to the file.

var base64Data = req.rawBody.replace(/^data:image\/png;base64,/, "");

require("fs").writeFile("out.png", base64Data, 'base64', function(err) {
  console.log(err);
});

new Buffer(…, ‘base64’) will convert the input string to a Buffer, which is just an array of bytes, by interpreting the input as a base64 encoded string. Then you can just write that byte array to the file.

Update

As mentioned in the comments, req.rawBody is no longer a thing. If you are using express/connect then you should use the bodyParser() middleware and use req.body, and if you are doing this using standard Node then you need to aggregate the incoming data event Buffer objects and do this image data parsing in the end callback.