Make slider value non-nullable

[thw0rted]

Documentation Feedback

The linked docs give the type of value as number | null but the overview never describes a state when the value can actually be null. They say "The initial value is set to the minimum value unless otherwise specified".

If the value actually can be null, an example of this should be provided. If the value is never null (which seems to make more sense?) then the API should be updated so that value is simply number.

Affected documentation page: https://material.angular.io/components/slider/api

[crisbeto]

I think that a null can be assigned here if the reset method of the related FormControl is called. Changing the type shouldn't be a problem, but it could be considered a breaking change at this point.

[thw0rted]

That's interesting, it didn't occur to me that the default for FormControl must always be null. Aren't there native controls whose value cannot be null? I feel like this came up on another issue recently but I'm having trouble remembering which.

ETA: might have been thinking of <input type=range> since the MDN page says

The value attribute contains a DOMString which contains a string representation of the selected number. The value is never an empty string ("").

(It's always a string, never null, never the empty string.)

[crisbeto]

We've handled similar cases in other components by setting it to zero when null is passed in. It's just that updating the type now would be breaking.