Sharepoint - Check if user is in a specified group

How to check if current user belongs to SharePoint group via CSOM (JavaScript):

function IsCurrentUserMemberOfGroup(groupName, OnComplete) {

        var currentContext = new SP.ClientContext.get_current();
        var currentWeb = currentContext.get_web();

        var currentUser = currentContext.get_web().get_currentUser();
        currentContext.load(currentUser);

        var allGroups = currentWeb.get_siteGroups();
        currentContext.load(allGroups);

        var group = allGroups.getByName(groupName);
        currentContext.load(group);

        var groupUsers = group.get_users();
        currentContext.load(groupUsers);

        currentContext.executeQueryAsync(OnSuccess,OnFailure);

        function OnSuccess(sender, args) {
            var userInGroup = false;
            var groupUserEnumerator = groupUsers.getEnumerator();
            while (groupUserEnumerator.moveNext()) {
                var groupUser = groupUserEnumerator.get_current();
                if (groupUser.get_id() == currentUser.get_id()) {
                    userInGroup = true;
                    break;
                }
            }  
            OnComplete(userInGroup);
        }

        function OnFailure(sender, args) {
            OnComplete(false);
        }    
}

Usage:

function IsCurrentUserHasContribPerms() 
{
  IsCurrentUserMemberOfGroup("Members", function (isCurrentUserInGroup) {
    if(isCurrentUserInGroup)
    {
        // The current user is in the [Members] group!
    }
  });

}
ExecuteOrDelayUntilScriptLoaded(IsCurrentUserHasContribPerms, 'SP.js');

Gist SP.IsCurrentUserMemberOfGroup.js

Here's a quicker way with SharePoint 2013:

function CheckCurrentUserMembership() {

    var clientContext = new SP.ClientContext.get_current();
    this.currentUser = clientContext.get_web().get_currentUser();
    clientContext.load(this.currentUser);

    this.userGroups = this.currentUser.get_groups();
    clientContext.load(this.userGroups);
    clientContext.executeQueryAsync(OnQuerySucceeded);
}

function OnQuerySucceeded() {
         var isMember = false;
         var groupsEnumerator = this.userGroups.getEnumerator();
          while (groupsEnumerator.moveNext()) {
             var group= groupsEnumerator.get_current();               
             if(group.get_title() == "Administrator Group") {
                 isMember = true;
                 break;
             }
          }

          OnResult(isMember);
}

function OnQueryFailed() {
          OnResult(false);
}

This code worked for me, its simple and does the job

var url = _spPageContextInfo.webAbsoluteUrl +'/_api/web/currentuser/groups'
    $.getJSON(url, function (data) {
        $.each(data.value, function (key, value) {
            if (value.Title == 'GroupName') {               
                 //*****your code goes here*****//
            }
        });
    })