{
    'userName': 'test@gmail.com',
    'password': 'Password!',
    'grant_type': 'password'
}

var obj = {
  method: 'POST',
  headers: {
    'Content-Type': 'application/x-www-form-urlencoded; charset=UTF-8',
  },
};
fetch('https://example.com/login', obj)
  .then(function(res) {
    // Do stuff with result
  });

var params = {
    userName: 'test@gmail.com',
    password: 'Password!',
    grant_type: 'password'
};

var formData = new FormData();

for (var k in params) {
    formData.append(k, params[k]);
}

var request = {
    method: 'POST',
    headers: headers,
    body: formData
};

fetch(url, request);

var details = {
    'userName': 'test@gmail.com',
    'password': 'Password!',
    'grant_type': 'password'
};

var formBody = [];
for (var property in details) {
  var encodedKey = encodeURIComponent(property);
  var encodedValue = encodeURIComponent(details[property]);
  formBody.push(encodedKey + "=" + encodedValue);
}
formBody = formBody.join("&");

fetch('http://identity.azurewebsites.net' + '/token', {
  method: 'POST',
  headers: {
    'Accept': 'application/json',
    'Content-Type': 'application/x-www-form-urlencoded'
  },
  body: formBody
})

var data = new URLSearchParams();
data.append('userName', 'test@gmail.com');
data.append('password', 'Password');
data.append('grant_type', 'password');

Object.prototype.serialize = function () {
  if (!this) return;

  var s = []

  for (var key in this) {
    if (this.hasOwnProperty(key)) {
      s.push(encodeURIComponent(key) + '=' + encodeURIComponent(this[key]))
    }
  }

  return s.join('&')
}

obj.serialize()

var data = {
  a: 1,
  b: 2,
  c: true
}

fetch('/api_url', {
  method: 'POST',
  headers: {
    'Accept': 'application/json',
    'Content-Type': 'application/x-www-form-urlencoded'
  },
  body: data.serialize() // see the usage here
})