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 t

Answer 1

It returns the first true from the left. If no true it returns false. If the expressions resolve to true, in case of a Boolean or a non-zero or non-null or non-undefined value.

Edit:

Yes, the value has to be truthy...not only true.

Answer 2

Your code doesn't work because you can't use the value in cache when it's 0 as 0 || func() asks for the function to be called.

So it always call the second term for 0 and thus makes a stack overflow as the recursion has no end.

A solution would be to change your internal function like this :

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

As an aside please note the spelling of Fibonacci.

Answer 3

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.