Selenium webdriver JS take screenshot -- from http://stackoverflow.com/questions/15902142/webdriverjs-selenium2-from-nodejs-correct-syntax-for-taking-screenshot

ss.js

function writeScreenshot(data, name) {
  name = name || 'ss.png';
  var screenshotPath = 'C:\\selenium_local_map\\';
  fs.writeFileSync(screenshotPath + name, data, 'base64');
};

driver.takeScreenshot().then(function(data) {
  writeScreenshot(data, 'out1.png');
});

Thanks for making this

Another way to do it, from the docs
var screenshot = browser.saveScreenshot(); // returns base64 string buffer
fs.writeFileSync('./myShort.png', screenshot)

It's not working anymore:

Failed: ENOENT: no such file or directory, open './screenshots/out1.png'

And from the comment:
browser.saveScreenshot() - that's a method from webdriver.io, not present in Selenium webdriver.