++{String}, prefix-operator, converting the right-side argument to a number

 The plus operator is defined for numbers and strings and as soon as a string is present on either the left or right side, a string concatenation is perfomed.

If we follow the execution path, this is what happens:

// 1st step
'b' + 'a' -> 'ba'
// 2nd step
'ba' + + 'a' // wait a second!

There are two plus operators in this expression. But one of those is actually a prefix-operator, and not a classical plus.
What it does is converting the right-side argument to a number, but converting 'a' to a number will actually yield NaN!
Let's continue with following the execution path:

// 1st step
'b' + 'a' -> 'ba'
// 2nd step
'ba' + + 'a' -> 'ba' + NaN -> 'baNaN'
// 3rd step
'baNaN' + 'a' -> 'baNaNa'
// 4th step
'baNaNa'.toLowerCase() -> 'banana'