SP.RoleDefinitionCollection object

<asp:Content ID="Main" ContentPlaceHolderID="PlaceHolderMain" runat="server">
<script type="text/ecmascript" language="ecmascript">

    var roleDefinitionCollection;
    var roleDefinition;
    function runCode() {
        var clientContext = new SP.ClientContext.get_current();
        if (clientContext != undefined && clientContext != null) {
            var web = clientContext.get_web();

            var permissionsManage = new SP.BasePermissions();
            permissionsManage.set(SP.PermissionKind.manageUsers);

            // Specify properties of the role definition.
            var roleCreationInfo = new SP.RoleDefinitionCreationInformation();
            roleCreationInfo.set_basePermissions(permissionsManage);
            roleCreationInfo.set_description('Role for managing user permissions.');
            roleCreationInfo.set_name('Manage User');
            roleCreationInfo.set_order(1);

            // Add to the role definitions collection.
            this.roleDefinitionCollection = web.get_roleDefinitions();
            this.roleDefinition = this.roleDefinitionCollection.add(roleCreationInfo);

            clientContext.load(roleDefinition);

            clientContext.executeQueryAsync(Function.createDelegate(this, this.onQuerySucceeded), Function.createDelegate(this, this.onQueryFailed));
        }
    }

    function onQuerySucceeded() {
        alert('Created role: ' + this.roleDefinition.get_name());
    }

    function onQueryFailed(sender, args) {
        alert('Request failed. ' + args.get_message() + '\n' + args.get_stackTrace());
    }

</script>

    <input id="Button1" type="button" value="Run Code" onclick="runCode()" />

</asp:Content>