Default data not re-added when removing property value

[jacobmischka]

The current behavior makes sense, but it would be great if default data could somehow be reinjected if previously-provided data gets removed. It can particularly get in the way when using spread for props.

REPL: https://svelte.technology/repl?version=2.6.1&gist=b2cd6467f026d0b3a0857d248d44f830

It may be sort of related to #1434, but I actually want the falsey values passed down, I just want the default data ones to re-take effect.

I guess I basically am wanting an implicit something like this:

onstate({ current }) { this.set(Object.assign(defaultData, current)); }

Edit: I guess using Object.assign for this doesn't work, becuase undefined is indeed a value which is overriding the defaults. I wonder if this is what's causing the issue in the first place.

Thanks!

[jacobmischka]

Actually, it looks like defaultData already is re-added, it's just that changed[key] isn't set to true when a property is removed.

Ha my mistake, I thought that for a minute because I didn't realize svelte treats null the same as '' for interp, while it treats undefined as 'undefined'.

[TehShrike]

@jacobmischka I think the misunderstanding here is that the data function doesn't return "default data" so much as it returns "initial data" for a component - the spread operator overwrites default in Nested there, but there's nothing that would cause the value of default to go back to 'here' once the spread operator doesn't have a value for that property and sets it back to undefined.

To get the behavior you want, a computed property is probably best. See this version, where Nested takes a value property, but references a computed property in the template in order to fall back to a default value: https://svelte.technology/repl?version=2.6.1&gist=0251ae7e167fc715a24d198d09aaaf51

[jacobmischka]

That makes sense. I suppose this is more of a feature request than a bug report, because I think it would be nice if it behaved that way. Though you're right, computed properties could work just as well, thanks for the suggestion.

[TehShrike]

Even if #1434 is implemented, I imagine it will be "don't touch the current values when undefined is passed in" and not "remember what the value was at some time in the past, and set it back to that value when the value is changed to undefined by some other code"

[Rich-Harris]

Going to close this as the behaviour is expected. In v3 it's a good deal easier to patch objects as opposed to replacing them — instead of

this.set({nestedData: { added: 'updated!' }});

it's possible to do

nestedData.added = 'updated!';

thus leaving nestedData.default untouched.