SOURCE

var dom = document.querySelector(".div1");
dom.onmousedown = function(event) {
        // 计算鼠标和原图相对距离
        var disX = event.clientX - dom.offsetLeft;
        var disY = event.clientY - dom.offsetTop;

    dom.onmousemove = function(e) {
        var x = e.clientX - disX;
        var Y = e.clientY - disY;
        dom.style.left = x + "px";
        dom.style.top = Y + "px";
     }
    dom.onmouseup = function() {
        dom.onmousemove = null;
        dom.onmouseup = null;
    }
}

<div class="div1">aaa</div>

.div1 {
    position: relative;
    cursor: e-resize;
}