FirstOrDefault

[NiallArkEnergy]

FirstOrDefault doesn't appear to work.
Enumerable.from([]).firstOrDefault(15) // null

[M0ns1gn0r]

Yep, its current implementation is broken. It doesn't expect that instead of a predicate, a default value can be passed as the first argument:

Enumerable.prototype.firstOrDefault = function (predicate, defaultValue) {
    if (defaultValue === undefined) defaultValue = null;
    if (predicate != null) return this.where(predicate).firstOrDefault(null, defaultValue);
    ...

I would fix it that way:

Enumerable.prototype.firstOrDefault = function (predicate, defaultValue) {
    if (predicate) {
        if (typeof predicate === Types.Function)
             return this.where(predicate).firstOrDefault(null, defaultValue);
        defaultValue = predicate;
    }
    defaultValue = defaultValue || null;
    ...

[NiallArkEnergy]

I like the change. Sorry I was an idiot and using the function incorrectly. I'm too used to javascript libraries doing the heavy lifting of checking input and adapting. C# isn't nearly as adaptable and this library is emulating C#

[M0ns1gn0r]

Don't blame yourself, the authors of linq.js changed the signature at some moment and introduced this breaking change. See http://neue.cc/reference.htm, here your code sample works as expected.