Enable ko.utils.extend to also extend Symbols

[bennieswart]

We often use symbols in binding contexts to store intermediate data used by bindings, and when creating a child binding context, these symbols should be copied along with the normal properties. This is done using ko.utils.extend underlying.

[bennieswart]

@mbest would you mind giving some feedback here? Do you think it is a valid use case?

[mbest]

I do see the value of copying symbol properties, but I'm concerned that this would be a breaking change.

[bennieswart]

@mbest what would be the protocol for merging this if it is a breaking change, albeit an improvement? In any case, I suspect the number of users that could possibly experience a break due to this would be minuscule.

PS Couldn't we get away with some low-probability breaking changes in 3.5?

[chrisknoll]

If all the existing tests work, isn't that enough to assert that it doesn't lead to a breaking change?

[uncaught]

This should not be the default behavior. All extending/merging/assigning methods of any library I know (underscore, jquery, lodash) or native code ignore the non-enumerable properties. So while I have actually needed to extend something with symbols before, I still think you should hide this feature behind a flag or use a different method name. It is just not the expected behavior of such a method.

[bennieswart]

I think @uncaught raises a valid point. The default behaviour for ko.utils.extend should perhaps be to exclude non-enumerable properties.
Still though, I think it's useful to have the option to include symbols when the binding context is extended. Perhaps we should hide this functionality in ko.utils.extend behind a flag and set that flag from the call to extend the binding context.

[the-djmaze]

I know, old habbits are hard to die in code when the standards use other namings like Object.assign().
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/assign

Object.getOwnPropertyNames doesn't list Symbol
Object.getOwnPropertySymbols does that

So what about 'extendWithSymbols' or 'assignWithSymbols'?