In the underscore.js library, specifically the very first function called forEach (alias for each), a test is made to check if the length property of the given object is an integer. The way it does this is not the traditional typeof check, but by using the plus operator
var foo = ;
Ready to run.
|Traditional number check
|Plus operator number check
You can edit these tests or add more tests to this page by appending /edit to the URL.