Programmatically Download sharepoint file

      Here I wil talk about how to download sharepoint file (image/Document/Video) on button click, SharePoint provides you inbuilt solution using STSNavigate function which helps to navigate to system page download.aspx and download.aspx contains code to download file

1. If using server side code

downloadButton.OnClientClick = "STSNavigate('" + SPContext.Current.Web.Url + "/_layouts/15/download.aspx?SourceUrl=" + fileURL + "' ); return false;";

In above line of code STSNavigate will do magic of giving user save as option of specified file URL, also here we are using system page download.aspx

2. If using client object model

if below code the only thing you need to do is to form fileURL.

$(‘#<%=btnDownload.ClientID%>’).click(function (e) {
STSNavigate(‘<%= SPContext.Current.Web.Url %>/_layouts/15/download.aspx?SourceUrl=’ + fileURL);

return false;
})