SOURCE

var wrap = document.querySelector(".captch")
var box = document.querySelector(".box")
var boxBg = document.querySelector(".box-bg")
var x = ''

box.addEventListener("mousedown",function(e){
    x = e.offsetX
    box.style.transition = 'none'
    boxBg.style.transition = 'none'
    box.addEventListener("mousemove",run)

})
box.addEventListener("mouseup",function(e){
    box.removeEventListener("mousemove",run)
    init()
})
box.addEventListener("mouseout",function(e){
    box.removeEventListener("mousemove",run)
    init()
})

function run(e){
    var left = ( e.offsetX + e.target.offsetLeft - x )
        if(left < 0 ){
            left = 0
        }
        if( left > 120 ){
            left = 120
        }
    boxBg.style.width = left + 'px'
    box.style.left = left  + 'px' 
}

function init(){
    if( parseInt( box.style.left ) <120 ){
        box.style.transition = 'all 0.3s linear 0s'
        boxBg.style.transition = 'all 0.3s linear 0s'
        setTimeout(function(){
            box.style.left = '0px'
            boxBg.style.width = '0px'
        },1)
    }
}

<div class="captch">
        <div class="box"></div>
        <div class="box-bg"></div>
    </div>

body{
    margin: 0;
    padding: 0;
}

.captch{
    width: 150px;
    height: 30px;
    border: 1px solid #999;
    position: relative;
    margin: 20px auto;
}

.box,.box-bg{
    width: 30px;
    height: 30px;
    background-color: #999;
    border: 1px solid #e8e8e8;
    box-sizing: border-box;
    position: absolute;
    top: 0;
    left: 0;
    cursor: pointer;
}
.box-bg{
    border: 0;
    width: 0px;
    background-color: #FF6666;
}