<script>
"use strict";

let formData = new FormData();
formData.append('key1', 'value1');
formData.append('key2', 'value2');

// List key/value pairs
for(let [name, value] of formData) {
  alert(`${name} = ${value}`); // key1 = value1, then key2 = value2
}
</script>