Handle NaN as input to a style function

[kronick]

Right now the binary search I wrote for style function interpolation will go into an infinite loop if passed a NaN as input. I think @lucaswoj fixed this for most use cases by validating stops in the style json before they are passed to interpolated().

But this check is not performed for other uses of this module (as would be desired to get rid of custom color interpolation code in https://www.mapbox.com/mapbox-gl-js/example/data-join/ . See also: https://github.com/mapbox/mapbox-gl-js/issues/4018). I have hit this problem in my own use of this function, and am now just checking isNaN() myself, but since the failure mode is an infinite loop I feel we should offer a better answer.

What should interpolated() return if passed a NaN? The value of the first stop (as is the default case in categorical functions)? The value of the last stop (as it looks like the old search would have done)? Throw an error? Something else?

I can update the function once I get some advice on what the desired output should be.

[jfirebaugh]

I think this falls under the same category as #588, and should have the same behavior as whatever we decide there -- probably, return undefined, until there's some means of specifying a fallback result as in #480.

[kronick]

@jfirebaugh Thanks-- I'll stand by until a decision is made there

[lucaswoj]

This issue was moved to mapbox/mapbox-gl-js#4172