Test assigning null to SVGAngle.valueAsString / SVGLength.valueAsString On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE". PASS angle.valueAsString is "0" PASS angle.valueAsString = null threw exception SyntaxError: The string did not match the expected pattern.. PASS angle.valueAsString is "0" PASS angle.valueAsString = '10' did not throw exception. PASS angle.valueAsString is "10" FAIL angle.valueAsString = '' should throw an exception. Was . PASS angle.valueAsString is "10" PASS length.valueAsString is "0" PASS length.valueAsString = null threw exception SyntaxError: The string did not match the expected pattern.. PASS length.valueAsString is "0" PASS length.valueAsString = '10' did not throw exception. PASS length.valueAsString is "10" FAIL length.valueAsString = '' should throw an exception. Was . PASS length.valueAsString is "10" PASS successfullyParsed is true TEST COMPLETE