unable to set property 'onclick' of undefined

Question

unable to set property 'onclick' of undefined and img

 

please help me

 

THIS is my modal.js file code

// Get the modal
var modal = document.getElementById('myModal');
// Get the image and insert it inside the modal - use its "alt" text as a caption
var img = document.getElementById('<%= ImgProfile.ClientID %>');
var modalImg = document.getElementById("img01");
//var captionText = document.getElementById("caption");
img.onclick = function () {
alert("hello");
modal.style.display = "block";
modalImg.src = this.src;
// captionText.innerHTML = this.alt;
}
// Get the <span> element that closes the modal
var span = document.getElementsByClassName("close")[0];

// When the user clicks on <span> (x), close the modal
span.onclick = function() {
modal.style.display = "none";
}

THIS is my home.aspx file code

<asp:Image ID="ImgProfile" runat="server" src="Images/19251082_10215259476544325_53831408_n.jpg" CssClass="img-thumbnail"/>
<div id="myModal" class="modal">
<span class="close">×</span>
<img class="modal-content" id="img01" />
<div id="caption"></div>
</div>