konwersja.js

var fileName = process.argv[2];

if (!fileName) {
    console.error('No file input!');
    return;
}

var Iconv = require('iconv').Iconv;
var fs = require('fs');

var charsetConverter = new Iconv("cp1250", "utf8");

fs.readFile(fileName, function (err, fileContent) {
    if (err) {
        throw err;
    }

    fs.writeFile(fileName, charsetConverter.convert(fileContent), function (err) {
        if (err) {
            throw err;
        }
        console.log('Done!');
    });
});