remove and add permissions to document library using JavaScript

function breakSecurityInheritanceAddUser(vals1) {    

	var itemId = _spPageContextInfo.pageItemId;	
	//alert(itemId);	
	var clientContext = SP.ClientContext.get_current();    
    var oList = clientContext.get_web().get_lists().getByTitle('Pages');
	this.oListItem = oList.getItemById(itemId);	
	//alert("0");
    oListItem.breakRoleInheritance(false, true);	
    while (oListItem.RoleAssignments.Count > 0)
	{
		item.RoleAssignments.Remove(0);
	}
	//alert("1");

	for(var j=0; j < vals1.length; j++){
	//	alert(vals1[j]);
		var groupCollection = clientContext.get_web().get_siteGroups();		
		this.oUser=groupCollection.getByName(vals1[j]);		
	//	alert("2");		
		var collRoleDefinitionBinding = SP.RoleDefinitionBindingCollection.newObject(clientContext);
		collRoleDefinitionBinding.add(clientContext.get_web().get_roleDefinitions().getByType(SP.RoleType.reader));
		//alert("3");		
		oListItem.get_roleAssignments().add(oUser, collRoleDefinitionBinding);
		clientContext.load(oUser);
		clientContext.load(oListItem);
		//alert("4");		
	    clientContext.executeQueryAsync(Function.createDelegate(this, this.onQuerySucceededRole), Function.createDelegate(this, this.onQueryFailedRole));
	}
		alert("Page permissions has been updated");
}

function onQuerySucceededRole(sender, args) {


//alert('Role inheritance broken for item ' + this.oListItem.get_item('Title') +' and new role assignment for ' + this.oUser.get_loginName());
}

function onQueryFailedRole(sender, args) {

  //  console.loge('Request failed. ' + args.get_message() + '\n' + args.get_stackTrace());
}

i am using above code to brake the permission inheritance and to add new SharePoint group to page library.

when am going add new SharePoint group i need to remove existing groups from from document library.

please help me