Read binary file and encode base64 for json member in node.js

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JSON

Base64

Node.js

Read binary file, encode it as base64 data and put this in json.
Display base64 in console.log (data.src) of readBinary function.
However, there is no src member in the resData JSON after calling the function.
How can I solve this?

What I have tried:

app.post('/server', function(req, res){
    var resData = {}; 

    /*return to resData JSON {num: 1, planet: 12, math: 234, int: 102}*/
    resData = prevInstance[num].getFile(num++);

    readBinary(resData, num);

    /*here no member src*/   
    console.log(resData);
    res.json(redData);
});

function readBinary(data, file_id){
    var fs = require('fs');            
    var bkgndBuffer = new Buffer.alloc(data.size, 'base64');   
        fs.open('projectfile.file', 'r', function(err, fd){
        fs.read(fd, bkgndBuffer, 0, size, position, function(){   
            /*adding 'src' member*/               
            data.src=bkgndBuffer.toString('base64');                       
            /* it's working. showing base64 data */
            console.log(data.src);
        });   
        fs.close(fd, function(){
            console.log("exit file read.");
        });                       
    }); 
}