Examples to retrieve a manager in Sharepoint 2013

method1a.js

// Retrieve Manager for a username
//   using REST API
var siteUrl = _spPageContextInfo.siteAbsoluteUrl;
var accountName = 'domain\\login';
// I suppose you use jQuery
$.ajax({
  url: siteUrl + "/_api/SP.UserProfiles.PeopleManager/GetPropertiesFor(accountName=@v)?@v='" + encodeURIComponent(accountName) + "'",
  method: "GET",
  headers: { "Accept": "application/json; odata=verbose" },
  success: function (data) {
    alert("The manager username is:" + data.d.ExtendedManagers.results.slice(-1)[0]);
  },
  error: function (data) {
    console.log(JSON.stringify(data));
  }
});

method1b.js

// Retrieve Manager for the current user
//   using REST API
var siteUrl = _spPageContextInfo.siteAbsoluteUrl;
$.ajax({
  url: siteUrl + "/_api/SP.UserProfiles.PeopleManager/GetMyProperties",
  method: "GET",
  headers: { "Accept": "application/json; odata=verbose" },
  success: function (data) {
    alert("The manager username is: " + data.d.ExtendedManagers.results.slice(-1)[0])
  },
  error: function (data) {
    console.log(JSON.stringify(data));
  }
});

method2a.js

// Retrieve Manager for a username
//   using SharepointPlus -- http://aymkdn.github.io/SharepointPlus/symbols/%24SP%28%29.html#.people
var username = "domaine\\login";
$SP().people(username, function(people) {
  alert("The manager username is: " + people["Manager"])
})

method2b.js

// Retrieve Manager for the current user
//   using SharepointPlus -- http://aymkdn.github.io/SharepointPlus/symbols/%24SP%28%29.html#.whoami
for the $SP().whoami(function(people) {
  alert("The manager username is: " + people["Manager"])
})