yes, it is not needed - explicit vs implicit choice.

a possibly more straightforward way would be

if ( !ingredients[recipeKeys[i]] ) {
    cakes = 0;
    break;
}

in the body of the loop, but +1 for documenting your approach

Clever one! :)

As i thought :)

No, it's not needed.

But is coercing it into a boolean needed, since javascript accepts truthy and falsy values?

The first coerces it to a boolean and the second reverts it to the proper value.

Along with what Jhonny said, i'd like to add that it's not good practice to write/modify the prototype property of an built-in objects.

Why are you using double negation in this one? Double negation implies the boolean itself.

!!x === x

It would be better to use a greedy regex operator because it's not safe to assume that there would be, at maximum, 3 "WUB"'s in a row.