Hi @tommy and @zhang_meifu
Thanks for pointing us for the right direction.
My first mistake was sending this POST request to the endpoint in text format It should be JSON request. 
Secondly, as @zhang_meifu mentioned in his reply, i had few issue with my index.js too. The issue was, reply from my javascript fetch request did not correctly passed to the ZoomMtg.join function. About hour of trial and error, i was able to fix my code finally.
Happy to say its working well now. Below is my index.js file. 
(function(){
console.log('checkSystemRequirements');
console.log(JSON.stringify(ZoomMtg.checkSystemRequirements()));
ZoomMtg.setZoomJSLib('https://dmogdx0jrul3u.cloudfront.net/1.4.2/lib', '/av');
ZoomMtg.preLoadWasm();
ZoomMtg.prepareJssdk();
var API_KEY = 'XXXXXXX';
document.getElementById('join_meeting').addEventListener('click', function(e){
e.preventDefault();
if(!this.form.checkValidity()){
alert("Enter Name and Meeting Number");
return false;
}
var meetConfig = {
apiKey: API_KEY,
meetingNumber: parseInt(document.getElementById('meeting_number').value),
userName: document.getElementById('display_name').value,
passWord: "",
leaveUrl: "https://zoom.us",
role: 0
};
var endpoint = 'https://ipaa.herokuapp.com';
var raw = JSON.stringify({"meetingNumber":meetConfig.meetingNumber,"role":1});
fetch(`${endpoint}`, {
method: 'POST',
headers: {
'Content-Type': 'application/json',
},
body: raw,
})
.then(result => result.text())
.then(response => {
console.log(response);
var signature = JSON.parse(response);
console.log(signature);
ZoomMtg.init({
leaveUrl: 'http://www.zoom.us',
isSupportAV: true,
success: function () {
ZoomMtg.join(
{
signature: signature.signature,
meetingNumber: meetConfig.meetingNumber,
userName: meetConfig.userName,
apiKey: meetConfig.apiKey,
//userEmail: 'user@gmail.com',
passWord: meetConfig.passWord,
success: function(res){
$('#nav-tool').hide();
console.log('join meeting success');
},
error: function(res) {
console.log(res);
}
}
);
},
error: function(res) {
console.log(res);
}
});
})
});
})();