Use chancejs github
email
chance.email()
chance.email({domain: "example.com"}) 
Return a random email with a random domain.
chance.email()
=> 'kawip@piklojzob.gov'
Optionally specify a domain and the email will be random but the domain will not.
chance.email({domain: 'example.com')
=> 'giigjom@example.com'
Or pure JavaScript
fiddle DEMO
function makeEmail() {
    var strValues = "abcdefg12345";
    var strEmail = "";
    var strTmp;
    for (var i = 0; i < 10; i++) {
        strTmp = strValues.charAt(Math.round(strValues.length * Math.random()));
        strEmail = strEmail + strTmp;
    }
    strTmp = "";
    strEmail = strEmail + "@";
    for (var j = 0; j < 8; j++) {
        strTmp = strValues.charAt(Math.round(strValues.length * Math.random()));
        strEmail = strEmail + strTmp;
    }
    strEmail = strEmail + ".com"
    return strEmail;
}
console.log(makeEmail());