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Mutable type guards #25679
Description
Search Terms
mutable type guards
Suggestion
Allow for mutable type guards (i.e. type guards that are always true).
Use Cases
I would like to extend HTMLElements without wrapping them. So basically I would add "extra" property to a HTMLElement that would extend its capabilities.
Examples
function setExtra<T>(el: T): el is T & { extra: { stuff: {} } } {
(el as any).extra = { stuff: {} }
}
const el = document.createElement('div')
el.extra // type is HTMLDivElement, therefore fail
setExtra(el)
el.extra // type is HTMLDivElement & { extra: { stuff: {} } }, therefore OK
Checklist
My suggestion meets these guidelines:
- This wouldn't be a breaking change in existing TypeScript / JavaScript code
- This wouldn't change the runtime behavior of existing JavaScript code
- This could be implemented without emitting different JS based on the types of the expressions
- This isn't a runtime feature (e.g. new expression-level syntax)
Activity
mhegazy commentedon Jul 16, 2018
you can achieve a similar pattern by slightly reordering your code to use immutable objects instead of mutations, e.g.
you can also achieve that using type assertions:
That siad, i do not think we have any plans to model side effects in the type system in the time being.
typescript-bot commentedon Jul 27, 2018
Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.
oldrich-s commentedon Sep 4, 2018
Mohamed Hegazy (@mhegazy) : I have run into the same problem again, this time with
MochaandChai:Chaithrows if thevaluedoes not exist, therefore after the assert function is called, the value should be of typenumber.For now, I have to manually retype values after each
assertcall, which is really not optimal.What solution do you propose to this problem?
RyanCavanaugh commentedon Sep 4, 2018
See #10421 and its linked issues
oldrich-s commentedon Sep 5, 2018
Thank you.
If I am not mistaken, the link you sent discusses block scope type assertions.
What I would like to do is to mutate the type of variables that are sent into a function as function parameters from inside of the function. That would probably break the "block scope" rule discussed in the provided link, right? Then I would have to do
Which is probably not optimal either.
Basically I would like to have type guards without the
ifstatement requirement.RyanCavanaugh commentedon Sep 5, 2018
See also #8655