Demo. Open the console!
<script>
var email = {
   from : "gabor@szabgab.com",
   to : [ "foo@bar.com", "qux@bar.com" ],
   subject : "Hello World",
}
console.log("hello")
console.log(email)
console.log(email["to"])
email["parts"] = []
email["parts"][0] = {
    "mime-type" : "text",
    body : "The content"
}
email["parts"].push({
    "mime-type" : "jpg"
})
console.log(email)
</script>