SP.SOD.executeFunc('sp.js', 'SP.ClientContext',
function (){
 //your code goes here...
 alert('script loaded');
      var context = new SP.ClientContext.get_current();
     var user = context.get_web().get_currentUser();
});

ExecuteOrDelayUntilScriptLoaded(init, "sp.js");

function init()
{
            this.clientContext = new SP.ClientContext.get_current();
            this.oWeb = clientContext.get_web();
            currentUser = this.oWeb.get_currentUser();
            this.clientContext.load(currentUser);
            this.clientContext.executeQueryAsync(Function.createDelegate(this, this.onQuerySucceeded), Function.createDelegate(this, this.onQueryFailed));

}