instanceof as QueryInterface shortcut

[Daniel Fournier]

Can someone explain me why using JavaScript instanceof operator as the 
side effect to convert its left operand to the type of its right operand.

An RDF example:

// rsc is an object of type nsIRDFNode

rsc instanceof Components.interfaces.nsIRDFLiteral;

// now resource can be used as an is an object of type nsIRDFLiteral
// same as
// rsc = rsc.QueryInterface(Components.interfaces.nsIRDFLiteral);
Is this shortcut safe?

Daniel