SOURCE

class Drag {
    constructor(el, parentBox) {
        this.dragBox = el;
        this.parentBox = parentBox;
        this.isMove = false;
        this.resizeBox = {}
        this.mouseDownHandler()
        this.createResizeBox()
        this.resizeHandler()
    }
    createResizeBox() {
        let resizeBox = document.createElement('div');
        resizeBox.style.width = '10px'
        resizeBox.style.height = '10px'
        resizeBox.style.background = 'green'
        resizeBox.style.position = 'absolute'
        resizeBox.style.cursor = 'se-resize'
        resizeBox.style.right = '0'
        resizeBox.style.bottom = '0'
        this.resizeBox = resizeBox
        console.log('resizeBox33333', resizeBox)
        this.dragBox.appendChild(resizeBox)
    }
    resizeHandler() {
        let self = this;
        this.resizeBox.onmousedown = function(evt) {
            evt.stopPropagation();
            evt.cancelBubble = true;
            let dragBoxWidth = self.dragBox.offsetWidth;
            let dragBoxHeight = self.dragBox.offsetHeight;
            let currentX = evt.clientX;
            let currentY = evt.clientY;
            document.onmousemove = function(e) {
                const rect = self.getOffsetByBody(self.parentBox)
                let disW = e.clientX;
                let disY = e.clientY;

                // 判断拖拽时宽高是否超出父元素

                if (e.clientX >= self.parentBox.offsetLeft + rect.offsetLeft + self.parentBox.offsetWidth) {
                    disW = self.parentBox.offsetLeft + rect.offsetLeft + self.parentBox.offsetWidth
                }
                if (e.clientY >= self.parentBox.offsetTop + rect.offsetTop + self.parentBox.offsetHeight) {
                    disY = self.parentBox.offsetTop + rect.offsetTop + self.parentBox.offsetHeight
                }
                let lastW = dragBoxWidth + disW - currentX;
                let lastH = dragBoxHeight + disY - currentY;
                self.dragBox.style.width = lastW >= 240 ? lastW + 'px' : 240 + 'px';
                self.dragBox.style.height = lastH >= 180 ? lastH + 'px' : 180 + 'px'
                document.onmouseup = function(evt) {
                    document.onmousedown = null;
                    document.onmousemove = null

                }
            }
        }
    }
    mouseDownHandler() {
        let self = this
        this.dragBox.onmousedown = function(evt) {
            self.isMove = true
            let rect = self.getOffsetByBody(self.dragBox)
            const offsetX = rect.offsetLeft;
            const offsetY = rect.offsetTop;
            const innerX = evt.clientX - offsetX - self.dragBox.offsetLeft
            const innerY = evt.clientY - offsetY - self.dragBox.offsetTop;

            document.onmousemove = function(evt) {
                let rect = self.getOffsetByBody(self.dragBox)
                let disX = evt.clientX - rect.offsetLeft - innerX
                let disY = evt.clientY - rect.offsetTop - innerY
                if (disX <= 0) {
                    disX = 0
                } else if (disX >= self.parentBox.offsetWidth - self.dragBox.offsetWidth) {
                    disX = self.parentBox.offsetWidth - self.dragBox.offsetWidth
                }
                if (disY <= 0) {
                    disY = 0
                } else if (disY >= self.parentBox.offsetHeight - self.dragBox.offsetHeight) {
                    disY = self.parentBox.offsetHeight - self.dragBox.offsetHeight
                }
                self.dragBox.style.left = disX + "px";
                self.dragBox.style.top = disY + "px";

                self.isMove = true
            }
            document.onmouseup = function(evt) {
                document.onmousedown = null;
                document.onmousemove = null
                self.isMove = false
            }
        }

    }
    getOffsetByBody(el) {
        let offsetTop = 0;
        let offsetLeft = 0;
        let ele = el.offsetParent;
        while (ele && ele.tagName !== 'BODY') {
            // console.log(ele, ele.tagName, ele.offsetLeft)
            offsetTop += ele.offsetTop;
            offsetLeft += ele.offsetLeft;
            ele = ele.offsetParent
        }
        return {
            offsetLeft,
            offsetTop,
        }
    }



}

var box = document.querySelector('#drag-box')
var parentBox = document.querySelector('#container')
// var dotBox = document.querySelector('#drag-dot')
new Drag(box, parentBox)

<div class="container-box" id="container">
    <div class="box" id="drag-box">
    </div>
</div>

 .box {
	width:240px;
	height:180px;
	background:red;
	position:absolute;
	left:10px;
	top:10px
}
.box:hover {
	cursor:move;
}
.container-box {
	width:800px;
	height:500px;
	border:1px solid blue;
	margin:0 auto;
	position:relative;
}
.box-dot {
	width:5px;
	height:5px;
	background:green;
	position:absolute;
	cursor:se-resize;
	right:0;
	bottom:0;
}