How to test existing JavaScript functions with mocha

I’m new to the mocha framework, I’m not completely getting out of it, I have set up my machine with that framework in visual studio 2017. I have below a sample piece of code however I’m not getting how to test the existing javascript code that is in .cshtml page that has page elements as well in it, please help.

   ///////////////////////////////////////////////
    //  SAmple Code of mocha
        var assert = require('assert');
        
        describe('Test Suite 1', function () {
            it('Test 1', function () {
                assert.ok(true, "This shouldn't fail");
            });
        
            it('Test 2', function () {
                assert.ok(1 === 1, "This shouldn't fail");
                assert.ok(false, "This should fail");
            });
        });
    ///////////////////////////////////////////////////////////////

// Our code inside .cshtml

function SerChr(val) {
var f = parseFloat(result1) + parseFloat(document.getElementById("body_SerPerInst").value)
document.getElementById("body_TotAmtPerInst").value = Math.floor(f);
totreturnamount = (parseFloat(num3) + (parseFloat(document.getElementById("body_TotAmtPerInst").value) * parseFloat(document.getElementById("body_TotInst").value)));
document.getElementById("body_TotRetAmt").value = totreturnamount;
var h = parseFloat(num2) + parseFloat(document.getElementById("body_SerPerInst").value);
document.getElementById("body_inst1").value = Math.floor(h);
}

2nd example

$("#form2").submit(function(e){
  e.preventDefault();
  if(validate()){

    var formData = new FormData($(this)[0]);
    $.ajax({
      url: "addv.php",
      type: 'POST',
      data: formData,
      success: function (data) {
        $('.fv').html(data).hide().fadeIn(500).delay(1000).fadeOut(500);
      },
      contentType: false,
      processData: false
    });     
  } else {
   alert('error');
  }
});

// Another example

function validate()
{
    var m=document.getElementById("mat").value;
    var c=document.getElementById("kilo").value;
    var v=true;
    if(!isNaN(m))
        {
          alert("matricule  doit etre une chaine !");
          document.getElementById("mat").value=null;
          v=false;
        }
    if(isNaN(c))
        {
          alert("kilométrage doit etre un entier !");
          document.getElementById("kilo").value=null;  
          v=false;
        }
       return v;
}