Why the condition based on setInterval's function doesn't run?

I'm just using a setInterval to animate my elements, and I aim to do something when index === 5, which is incrementing inside the setInterval's function,

but I can't refer to that index to console.log() something or do whatever, how should I do that?

    let index = 0;
    function slideAction(nth) {
        console.log(index)
        index++;
    }
    let IntervalID = null;
    function slideActionIntervalManager(flag, slideAction, nth) {
        if (flag){
            intervalID = setInterval(function () { slideAction(nth); }, 1000);
        }
        else
            clearInterval(intervalID);
    }
    function firstStartSlide (){
        slideActionIntervalManager(true,slideAction, 1)
        if (index === 5){
            console.log('if true')
        }
        else if (index === 9){
            alert()
        }
    }
    firstStartSlide();

*{
            padding: 0;
            margin: 0;
        }

        /*---  slideWrapperContainer  ---*/
        .main{
            margin: 100px auto;
            border: 5px solid #000;
            position: relative;
            width: 700px;
            height: 360px;
            /*overflow: hidden;*/
        }

        /*-------  slideWrapper  ------*/
        .outer-img-wrapper{
            width: 100%;
            height: 100%;
        }

        /*----------  slide  ----------*/
        .img-wrapper{
            border: 2px solid red;
            position: absolute;
            top: 0;
            left: 100%;
            height: 360px;
            width: 3500px;
        }
        img{
            height: 100%;
        }

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>