A simple USSD simulator using fetch, prompt and alert. Can be easily adapted to any spec.

USSDSimulator.html

<input type="text" id="phone" placeholder="0200672215" />
<button class="btn" id="sendBtn">Send</button>



<script>
    var input = document.getElementById('phone');

    var makeRequest = function(messageType, sessionId, message) {
        fetch("/ussd", {
            method: "POST",
            headers: {
            },
            body: JSON.stringify({
                messageType: messageType,
                sessionId: sessionId,
                msisdn: input.value,
                message: message
            }),
        })
        .then(response => response.json())
        .then(response => {
            if(response.responseType == "continue") {
                var msg = prompt(response.response);
                makeRequest(msg !== null ? "continue" : "end", sessionId, msg);
            }
            else {
                alert(response.response);
            }
        })
        .catch(err => alert("Something went wrong"));
    }

    function makeid(len) {
        var text = "";
        var possible = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";

        for (var i = 0; i < len; i++)
        text += possible.charAt(Math.floor(Math.random() * possible.length));

        return text;
    }


    var btn = document.getElementById('sendBtn');
    btn.onclick = function() {
        var sessionId = makeid(10);
        makeRequest("begin", sessionId);
    }

</script>