Use .map instead of a for loop

map-loop.js

// WRONG
var mixedEmails = ['JOHN@ACME.COM', 'Mary@FooBar.com', 'monty@spam.eggs'];

var tempList = [];

for (var i = 0; i < mixedEmails.length; i++) {
    tempList.push(mixedEmails[i].toLowerCase());
}

document.getElementsByTagName('body')[0].innerHTML += 'Old method: ' + tempList;

// RIGHT
var mixedEmails = ['JOHN@ACME.COM', 'Mary@FooBar.com', 'monty@spam.eggs'];
var validEmails = [];

function toLower(str) {
    return str.toLowerCase();
}

validEmails = mixedEmails.map(toLower);

document.getElementsByTagName('body')[0].innerHTML += '<br><br>Right method: ' + validEmails