isFinite() is a top-level function (like isNaN()) that “Evaluates an argument to determine whether it is a finite number.” JFSIII pointed this out to me, debugging my code. I got lazy checking a numeric value was not undefined and had something like this:

// check if value is defined
if ( value ) {
  // do stuff..
}

The problem was that 0 would be a proper value, but it is evaluated as falsey in the above conditional.

!!0
// >> false
!!isFinite(0)
// >> true

// check if value is numeric
if ( isFinite(value) ) {
  // do stuff
}

isFinite() does not specifically check if the value is a number, as it returns true for booleans.

isFinite('foo')
// >> false
isFinite(function(){})
// >> false
isFinite(true)
// >> true
isFinite(false)
// >> true