Math.random array length +1?

I don't understand something in this code :

vars.randAnim = anims[Math.floor(Math.random()*(anims.length + 1))];

Let's say we have a new Array, with 5 elements, so from index 0 to index 4

Then let's say Math.random() returns 1. So, we've got the floor of 1*5, but the 5 index doesn't exist in my Array, plus here we add a "+1"

Why +1? I would have expected exactly the opposite with "-1".

The code is wrong (as Rocket said). It should be

Math.floor(Math.random()*(anims.length))

But also the reasoning has a flaw : Math.random() cannot return "1".

Math.random does not return 1, it returns a value from 0 to 1. Math.floor basically truncates the decimal off of the value.

So, if Math.random returns 0.3984753005206585 and multiply it by 4 we get 1.593901202082634 which becomes 1 after Math.floor.

When you want to get a random value from an array, you multiply Math.random by the length, so it will return values between 0 and the length (but never the length).

The anims.length + 1 is incorrect in the above code.