Underscore.js, why does `isFunction` use `|| false`?

Question

The optional override for isFunction(object) in Underscore.js (repo link to definition), reads as follows:

// Optimize `isFunction` if appropria         


        

Answer 1

See the issues covered in the comments, #1621, #1929 and #2236.

Shortly put, some platforms have a bug where typeof isn't a string unless you store it in a variable.
The || false fixes the issue without introducing an extra variable.

Taken directly from #1621:

In IE8, with a variable everything works as expected:

var t = typeof obj
t === 'function' // false
t === 'object' // true

but without one, things are different:

(typeof obj) === 'function' // true, but SHOULD be false
(typeof obj) === 'object' // true

The extra check outlined above fixes the bug.