createTransformer does not support transformer function that has an argument with a default value (ES6)

[timmolendijk]

createTransformer takes a transformer argument, which is expected to be a function that takes precisely one argument. MobX tries to test this requirement by checking that transformer.length === 1.

The problem though is that this will break in the case of functions that have an argument with a default value, because those will have length === 0 (at least in Node 5 and in latest Google Chrome).

For example, this breaks (and unnecessarily so as far as I can see):

const slice = createTransformer((data = []) => data.slice(0, 5));

[mweststrate]

Closed as the fix will be part of the upcoming 3.0 release