How to add data to an event or user property using js dictionary

I am working on implementing mixpanel tracking for the first time. I am tracking events/users correctly and they are showing up within the live view just fine.

However when I assign a value to a property (say $email) from a dictionary by accessing the key that I’m interested in, no data is sent to mixpanel about that property. I have only successfully sent property data to mixpanel when assigning a variable that was assigned to by accessing the key from the dictionary. 

Here’s a sample script that I set up demonstrating what I wrote above.  Is this approach not possible? Will I need to assign the value of the key that I’m interested to a variable each time and then assign that variable to the property I’m interested in? 

Thanks for your time in helping with this.  

<script>

var newUserDict = {
    "email" : "adrian@awesome.com",
    "howHeard" : "facebook"
}

var emailVar = newUserDict["email"]
var howHeardVar = newUserDict["howHeard"]

console.log("email var " + emailVar);
console.log("howHeard var " + howHeardVar);

var mixpanelButton = document.getElementById("test-button")
mixpanelButton.setAttribute('onClick', 'AccountCreated("'+newUserDict+'")')

function AccountCreated(newUserDict) {

        mixpanel.track("Sign Up Test",{
        //not successfully sending to mp
         "$email": newUserDict["email"],
        "How Heard": newUserDict["howHeard"],
        "email222": newUserDict.email,
        "How Heard222": newUserDict.howHeard,
        //successfully sending to mp    
    "email3": emailVar,
    "hh": howHeardVar
    });
}

</script>