Sometimes you can prove that values will remain integer when iterating over array indices for example. However, in the general case, when doing integer additions or multiplications, they need to insert dynamic integer overflow check instructions everywhere. When the integers would overflow they have to be converted to floating-point values.

If you think about this for a second. Suppose I have a loop where I'm multiplying values:

function foo(x, y, n) { for (var i = 0; i < n; ++i) { x = x * y; }

    return x;

}

In order to know that the multiplication won't overflow, you have to be able to prove that both x and y, coming into that function, will be integers. You also have to have information about the range of x, y and i. If you know that x>=0 and y>= but you don't know how many times the loop will execute at compile time, you are basically screwed. You could unroll the loop to reduce the number of dynamic checks you need, but then that increases your code size. So you mostly have to check that the multiplication doesn't overflow on every iteration. You may also have to do dynamic type checks if you can't prove that x,y,i will always be integers.