There is no need to pass the 'driver' instance around, and not as many parentheses.

Using WD.js

function login(username, password) {
    return this
        .waitForElementByCss('.loginForm', asserters.isDisplayed, seconds(explicit_wait))
        .elementById('emailAddress').sendKeys(username)
        .elementById('password').sendKeys(password)
        .elementById('signInButton').click()
}

Using WebDriverJS

function login(driver, username, password) {
    driver.wait(until.elementIsVisible(driver.findElement({css: '.loginForm'})),    seconds(explicit_wait));
    driver.findElement({id: 'emailAddress'}).sendKeys(username);
    driver.findElement({id: 'password'}).sendKeys(password);
    driver.findElement({id: 'signInButton'}).click();
}