CODE EXAMPLE FOR JAVASCRIPT

callback() function with example

const checkfile = (x,callback)=>{
    if(typeof x !== 'number')
    return callback('not a number'); //(err,{})
    
    callback(false,'yes it is');
}


checkfile(4,(err, data)=>{
    if(err)
    console.log(err);
    else 
    console.log(err);
})