SharePoint JS: SP.File.checkOutType property gets a value that indicates how the file is checked out of a document library.

file-checkedout-csom.js

//Get file checkout type via CSOM (JavaScript)

var context = SP.ClientContext.get_current();
var web = context.get_web();
var file = web.getFileByServerRelativeUrl(pageUrl);
context.load(file);

context.executeQueryAsync(
     function(){
         if(file.get_checkOutType() == SP.CheckOutType.online) {
                        console.log('The file is checked out');
         }    
      },
      function(sender, args){    
          console.log(args.get_message());
      }
);

file-checkedout-REST.js

//Get file checkout type via REST

$.ajax({url: "/_api/web/getFileByServerRelativeUrl('" + pageUrl  + "')/checkOutType",
        headers: { "Accept": "application/json; odata=verbose" }, 
        success: function(data) {
                  if(data.d.CheckOutType == 0) {
                        console.log('The file is checked out');
                  }
               }
});

filed-checkedout-state.js

function isCheckedOut() {
    var isCheckedOut = false;
    if (typeof(PageState) != "undefined" && PageState)
    {
        isCheckedOut = PageState.ItemIsCheckedOutToCurrentUser == "1";
    }
    if (!isCheckedOut)
    {
        //do redirect
    }
}