Renames all files in a folder with Node/Javascript

file_rename.js

var fs = require( 'fs' );
var path = require( 'path' );
// In newer Node.js versions where process is already global this isn't necessary.
var process = require( "process" );

var folder = path.join(__dirname,"/folder_name");
console.log(folder);

// Loop through all the files in the temp directory
fs.readdir( folder, function( err, files ) {
        if( err ) {
            console.error( "Could not list the directory.", err );
            process.exit( 1 );
        } 
        console.log("testing");
        files.forEach( function( file, index ) {
                // Make one pass and make the file complete
                //var temp= file.split("_");
                var t = file.toLowerCase();
                //console.log(file);
                //console.log(t.join("_") + "\n");
                var oldName = path.join(folder, file);
                var newName = path.join(folder,t);

                fs.rename(oldName, newName, function(error){
                    if (error) {
                        console.log("ERRRRRRRRRROOO");
                    }
                    else {
                        console.log(t + " success");
                    }
                });
             
        } );
} );