On the problem of finger sliding on the mobile end

structure: 

<div class="wrap">
    <div class="inner">   //touch
        //
          
    </div>
</div>

the height and width of wrap is fixed, and the width of inner is 100%

. 
//touchStart
var startY=event.pageY   //
//touchMove
var endY = event.pageY  //
var num = startY-endY   //

then set the transform:translateY (num) value of inner to slide up and down inner, but now I encounter a problem is how to determine the bottom / top of the slip stops sliding. I have tried that the offset of offsetTop relative to the parent element has no effect, it has always been 0 (but transform will not change this value? )
I hope all of you will give us some ideas and help with the confusion as you pass by. Thank you

.
Javascript
Mar.28,2021

Hello! Provide a way of thinking.
since you use translateY to slide up and down, you can use a variable to record the translateY value of the element, and the touchmove event adds the sliding distance of the phone to the original translateY. Here is a simple example that you can take a look at. Where css () is a function that sets and gets the value of the transform attribute. 

var startPoint = 0;
var startEl = 0;
inner.addEventListener('touchstart', function(e) {
    startPoint = e.changedTouches[0].pageY;
    startEl = css(inner, 'translateY');
});
inner.addEventListener('touchmove', function(e) {
    var nowPoint = e.changedTouches[0].pageY;
    var dis = nowPoint - startPoint;
    var translateY = dis + startEl;
    css(inner, 'translateY', translateY);
});

function css(element, attr, val){
    if(!element.transform){
        element.transform = {};
    }    
    if(typeof val === "undefined"){ 
        if(typeof element.transform[attr] === "undefined"){
            switch(attr){
                case "scale":
                case "scaleX":
                case "scaleY":
                case "scaleZ":
                    element.transform[attr] = 100;
                    break;
                default:
                    element.transform[attr] = 0;    
            }
        } 
        return element.transform[attr];
    } else {
        element.transform[attr] = val;
        var transformVal  = "";
        for(var s in element.transform){
            switch(s){
                case "scale":
                case "scaleX":
                case "scaleY":
                case "scaleZ":
                    transformVal += " " + s + "("+(element.transform[s]/100)+")";
                    break;
                case "rotate":
                case "rotateX":
                case "rotateY":
                case "rotateZ":
                case "skewX":
                case "skewY":
                    transformVal += " " + s + "("+element.transform[s]+"deg)";
                    break;
                default:
                    transformVal += " " + s + "("+element.transform[s]+"px)";                
            }
        }
        element.style.WebkitTransform = element.style.transform = transformVal;
    }
}

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