Working with out parameters

When working with XPCOM components, you might come across method declarations like the following one:

[scriptable, uuid(8B5314BC-DB01-11d2-96CE-0060B0FB9956)]
interface nsITransferable : nsISupports {
  void getTransferData ( in string aFlavor, out nsISupports aData, out unsigned long aDataLen ) ;

The getTransferData method takes three parameters, aFlavor, aData, and aDataLen, and returns nothing. aData and aDataLen are marked as out, meaning that they act as "return values" for this method, and are changed during the method call. These are so-called out parameters.


In order to use such a method from JavaScript via XPConnect, you have to follow a specific rule. To get at the out parameters, you have to pass in an object. After the call, this object will have a new property called value, which contains the out values.

Assuming you have an object called transferable, you would invoke getTransferData() as follows:

var aData    = {};
var aDataLen = {};

transferable.getTransferData("text/unicode", aData, aDataLen);

var data    = aData.value;
var dataLen = aDataLen.value;

As you can see, after the call to getTransferData(), the out values are then contained in the value properties of aData and aDataLen.


When implementing a method which has out parameters in JavaScript, you have to set a new property called value to the out parameter which will hold the required value.

You would implement getTransferData() as follows:

 getTransferData: function(aFlavor, aData, aDataLen) {
   aData.value = resultData;
   aDataLen.value = resultData.length;

See also