cannot instantiate var camlQuery = new SP.CamlQuery(); object

my problem is when i try to instantiate an object of type CamlQuery() javascript compiler stop there, and could not continue, then i figured out that the javascript does not know the sharepoint javascript client api. i don't know what is missed:

            var camlQuery = new SP.CamlQuery();

            "camlQuery.set_viewXml("<View><Query>" +
                ......
            "</Query><RowLimit>50</RowLimit></View>");