the first error report is Uncaught TypeError: document.body.keydown is not a function
document.body .keydown is not a function, not document.body

secondly, keydown () is an instance method of a jQuery object, not a method on an dom instance. And to be honest, if the jQuery event binding specification is a little bit, it should be written as

$("body").on("keydown",function(e){
    //
})

so
we should write

document.querySelector("body").addEventListener("keydown",function(e){
    //
})

in order to have a chance to unbind later, it is best to give function a separate reference rather than an anonymous function

document.body.onkeydown = function(){
   if (event.keyCode == "13") {//keyCode=13
       document.getElementById('btnSumit').click();
   }
}

Native js writing directly requires the on prefix. onkeydown , onclick

if it's addEventListener , you don't need on prefix

document.body.onkeydown = function(){
   if (event.keyCode == "13") {//keyCode=13
       document.getElementById('btnSumit').onclick();
   }
}

------

document.body.addEventListener("keydown",function(){
   if (event.keyCode == "13") {//keyCode=13
       document.getElementById('btnSumit').onclick();
   }
},false)