In fact it is written in the comment right above:

// NaN means non-codepoint

So it is mandatory to perform this comparison first to handle the NaN case as in JavaScript:

NaN === NaN returns false .

As pointed out by James Wiseman it is also important to know why the developer used high !== high instead of isNaN(high) which would have been clearer.

It is certainly based on performance. This test shows that a !== a is twenty times faster than isNaN(a) .

zzzzBov also indicates that isNaN() could be overwritten, using !== is also more portable.

More info from Benjamin Gruenbaum:

It is also worth noting that NaN does not equal to anything else as well, and also it is not equal to anything else in an unstrict sense

And from Jan Dvorak: