SOURCE

// 获取模态窗口
var modal = document.getElementById('myModal');

// 获取图片模态框，alt 属性作为图片弹出中文本描述
var img = document.getElementById('myImg');
var modalImg = document.getElementById("img01");
var captionText = document.getElementById("caption");
img.onclick = function() {
    modal.style.display = "block";
    modalImg.src = this.src;
    modalImg.alt = this.alt;
    captionText.innerHTML = this.alt;
}

// 获取 <span> 元素，设置关闭模态框按钮
var span = document.getElementsByClassName("close")[0];

// 点击 <span> 元素上的 (x), 关闭模态框
span.onclick = function() {
    modal.style.display = "none";
}

<img id="myImg" src="http://www.runoob.com/wp-content/uploads/2016/04/img_lights.jpg"
alt="Northern Lights, Norway" width="300" height="200">
<!-- The Modal -->
<div id="myModal" class="modal">
    <span class="close">×</span>
    <img class="modal-content" id="img01">
    <div id="caption">
    </div>
</div>

#myImg {
    border-radius: 5px;
    cursor: pointer;
    transition: 0.3s;
}

#myImg:hover {
    opacity: 0.7;
}

/* The Modal (background) */
.modal {
    display: none;
    position: fixed;
    z-index: 1;
    padding-top: 100px;
    left: 0;
    top: 0;
    width: 100%;
    height: 100%;
    overflow: auto;
    background-color: rgba(0, 0, 0, 0.8);
}

/* Modal Content (image) */
.modal-content {
    margin: auto;
    display: block;
    width: 80%;
    max-width: 700px;
}
/* Caption of Modal Image */

#caption {
    margin: auto;
    display: block;
    width: 80%;
    max-width: 700px;
    text-align: center;
    color: #ccc;
    padding: 10px 0;
    height: 150px;
}


/* Add Animation */
.modal-content,#caption {
    -webkit-animation-name: zoom;
    -webkit-animation-duration: 0.6s;
    animation-name: zoom;
    animation-duration: 0.6s;
}
@-webkit-keyframes zoom {
    from {
        -webkit-transform: scale(0)
    }
    to {
        -webkit-transform: scale(1)
    }
}
@keyframes zoom {
    from {
        transform: scale(0.1)
    }
    to {
        transform: scale(1)
    }
}
/* The Close Button */
.close {
    position: absolute;
    top: 15px;
    right: 35px;
    color: #f1f1f1;
    font-size: 40px;
    font-weight: bold;
    transition: 0.3s;
}
.close:hover,
.close:focus {
    color: #bbb;
    text-decoration: none;
    cursor: pointer;
}
/* 100% Image Width on Smaller Screens */
@media only screen and (max-width: 700px) {
    .modal-content {
        width: 100%;
    }
}