function doSomeAsynchronousOperation(req,res,callback){
    if(req.body.username === ''){
        callback(new Error('user name empty'),false);
    }  
    callback(null,true);    
}
/**
 * callback are mostly used in Node.js
 * callback delivers an event asynchronously.
 * The user passes you a function (the callback), 
 * and you invoke it sometime later when the asynchronous operation completes. 
 * The usual pattern is that the callback is invoked as callback(err, result), 
 * where only one of err and result is non-null, 
 * depending on whether the operation succeeded or failed.
 */
callback are mostly used in Node.js
callback delivers an event asynchronously.
The user passes you a function (the callback),
and you invoke it sometime later when the asynchronous operation completes.
The usual pattern is that the callback is invoked as callback(err, result),
where only one of err and result is non-null,
depending on whether the operation succeeded or failed.
callback are mostly used in Node . js callback delivers an event asynchronously . The user passes you a function ( the callback ) and you invoke it sometime later when the asynchronous operation completes . The usual pattern is that the callback is invoked as callback ( err result ) where only one of err and result is non - null depending on whether the operation succeeded or failed .
function doSomeAsynchronousOperation(req,res,callback){
    if(req.body.username === ''){
        callback(new Error('user name empty'),false);
    }  
    callback(null,true);    
}
function doSomeAsynchronousOperation ( req , res , callback ) { if ( req . body . username === '' ) { callback ( new Error ( 'user name empty' ) , false ) ; } callback ( null , true ) ; }
callback are mostly used in Node.js
 callback delivers an event asynchronously.
callback are mostly used in Node . js callback delivers an event asynchronously .