My personal view on this is that using underscore is quite a pain, as it hurt my typing speed. I much prefer to use capital letter to 2 and + words.
e.g.:
leftPosition
hasMappingQuality
I know this is also “against” the Stylistic conventions, but I find it both faster to type and more concise. The most important I guess is to not use a capital letter at the beginning like HasMappingQuality since it would conflict against Modules and Types conventions.