Proper use of ||

Question

The general question I suppose is: when does || return the item on the left, and when does it return the item on the right?

The specific question, is why doesn't this work:

var fibonacci = (function () {

    var cache = [0, 1];

    function fibonacci(number) {
        return cache[number] = cache[number] || (fibnonacci(number - 1) + fibonacci(number - 2));
    }

    return fibonacci;
})();

var $div = $('div');

for (var index = 0; index < 10; index++) {
    $('<span />').text(fibonacci(index))
        .appendTo($div);
}

Answer 1

It returns the item on the left if and only if it is truthy.

The following are not truthy:

• The primitive boolean value false
• The primitive string value "" (the empty string)
• the numbers +0, -0 and NaN
• the primitive value null
• the primitive value undefined

Everything else is truthy.

Here is the list on the language specification.

In your case cache[0] returns 0 which as we can see is falsy so it enters recursion. This is why we avoid || for short circuiting in these situations.

You should consider checking directly that the object has that property: number in cache is one such way and another is cache[number] !== undefined.