Test for validationMessage IDL attribute for On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE". No message PASS testIt("", "", "") is "" Value missing PASS testIt("", "", "") is "value missing" Type mismatch PASS testIt("foo", "", "") is "" Range overflow PASS testIt("1982-11-02", "", "1970-12-31") is "range overflow" Range underflow PASS testIt("1982-11-02", "1990-05-25", "1990-12-24") is "range underflow" Step mismatch PASS testIt("1982-11-02", "", "", "123") is "step mismatch" PASS successfullyParsed is true TEST COMPLETE