Question

Please explain what this is doing with regards to calling a function and using the && operator.

callback(data && data.length > 1 && JSON.parse(data));

Also, which value actually gets passed to the function?

Answer 1

It is a way to say if data is true(not null) and its length is > 1 then call JSON.parse(data).
If first expression is true then only second expression is evaluated and so on.
Its equivalent to

if(data)

if(data.length > 1)

callback(JSON.parse(data));