Is it true that dictionaries can't have a default value specified?

[bzbarsky]

There's informative text in https://heycam.github.io/webidl/#idl-operations that says:

Dictionary types cannot have a default value specified explicitly

however it seems to me that null would be a valid default value for a dictionary (meaning empty dictionary, once you go through the es-to-dictionary steps).

@tobie @domenic @marcoscaceres @annevk

[annevk]

I think we decided not to have a default value (e.g., {}) since it always has a default value.

[bzbarsky]

What always has a default value?

In the current state of the IDL spec, optional dictionary arguments in trailing position always have a default value. I think that non-trailing-position ones should too. But dictionary-typed members of dictionaries do not have a default value, and there are specs relying on that at this point. Firefox used to give them a default value and we just stopped doing that because of those specs. See #76. However some other specs do want to default those to empty dictionary, I suspect. Hence this issue.

[bzbarsky]

For example, DOMQuadInit at https://drafts.fxtf.org/geometry/#dictdef-domquadinit sure seems to assume that its DOMPointInit members get default values. It has no provisions for those members being missing.

@zcorpan

[zcorpan]

Good catch. Yeah, I think I assumed that they would act like {}.

The other option is to default with prose in the algorithm, though that seems less nice.