[javascript] How do I make an input field accept only letters in javaScript?

var selectedRow = null;

function validateform() {

  var table = document.getElementById("mytable");
  var rowCount = table.rows.length;
  console.log(rowCount);

  var x = document.forms["myform"]["usrname"].value;
  if (x == "") {
    alert("name must be filled out");
    return false;
  }

  var y = document.forms["myform"]["usremail"].value;
  if (y == "") {
    alert("email must be filled out");
    return false;
  }
  var mail = /[^@]+@[a-zA-Z]+\.[a-zA-Z]{2,6}/
  if (mail.test(y)) {
    //alert("email must be a valid format");
    //return false ;
  } else {
    alert("not a mail id")
    return false;
  }

  var z = document.forms["myform"]["usrage"].value;
  if (z == "") {
    alert("age must be filled out");
    return false;
  }

  if (isNaN(z) || z < 1 || z > 100) {
    alert("The age must be a number between 1 and 100");
    return false;
  }

  var a = document.forms["myform"]["usrdpt"].value;
  if (a == "") {
    alert("Dept must be filled out");
    return false;
  }

  var dept = "`@#$%^&*()+=-[]\\\';,./{}|\":<>?~_";
  if (dept.match(a)) {
    alert("special charachers found");
    return false;
  }

  var b = document.forms["myform"]["usrclg"].value;
  if (b == "") {
    alert("College must be filled out");
    return false;
  }
  console.log(table);
  var row = table.insertRow(rowCount);
  row.setAttribute('id', rowCount);
  var cell0 = row.insertCell(0);
  var cell1 = row.insertCell(1);
  var cell2 = row.insertCell(2);
  var cell3 = row.insertCell(3);
  var cell4 = row.insertCell(4);
  var cell5 = row.insertCell(5);
  var cell6 = row.insertCell(6);
  var cell7 = row.insertCell(7);


  cell0.innerHTML = rowCount;
  cell1.innerHTML = x;
  cell2.innerHTML = y;
  cell3.innerHTML = z;
  cell4.innerHTML = a;
  cell5.innerHTML = b;
  cell6.innerHTML = '<Button type="button" onclick=onEdit("' + x + '","' + y + '","' + z + '","' + a + '","' + b + '","' + rowCount + '")>Edit</BUTTON>';
  cell7.innerHTML = '<Button type="button" onclick=deletefunction(' + rowCount + ')>Delete</BUTTON>';

}

function emptyfunction() {
  document.getElementById("usrname").value = "";
  document.getElementById("usremail").value = "";
  document.getElementById("usrage").value = "";
  document.getElementById("usrdpt").value = "";
  document.getElementById("usrclg").value = "";
}

function onEdit(x, y, z, a, b, rowCount) {
  selectedRow = rowCount;
  console.log(selectedRow);
  document.forms["myform"]["usrname"].value = x;
  document.forms["myform"]["usremail"].value = y;
  document.forms["myform"]["usrage"].value = z;
  document.forms["myform"]["usrdpt"].value = a;
  document.forms["myform"]["usrclg"].value = b;
  document.getElementById('Add').style.display = 'none';
  document.getElementById('update').style.display = 'block';
}

function deletefunction(rowCount) {
  document.getElementById("mytable").deleteRow(rowCount);
}

function onUpdatefunction() {
  var row = document.getElementById(selectedRow);
  console.log(row);

  var x = document.forms["myform"]["usrname"].value;
  if (x == "") {
    alert("name must be filled out");
    document.myForm.x.focus();
    return false;
  }

  var y = document.forms["myform"]["usremail"].value;
  if (y == "") {
    alert("email must be filled out");
    document.myForm.y.focus();
    return false;
  }

  var mail = /[^@]+@[a-zA-Z]+\.[a-zA-Z]{2,6}/
  if (mail.test(y)) {
    //alert("email must be a valid format");
    //return false ;
  } else {
    alert("not a mail id");

    return false;
  }

  var z = document.forms["myform"]["usrage"].value;
  if (z == "") {
    alert("age must be filled out");
    document.myForm.z.focus();
    return false;
  }
  if (isNaN(z) || z < 1 || z > 100) {
    alert("The age must be a number between 1 and 100");
    return false;
  }

  var a = document.forms["myform"]["usrdpt"].value;
  if (a == "") {
    alert("Dept must be filled out");
    return false;
  }
  var letters = /^[A-Za-z]+$/;
  if (a.test(letters)) {
    //Your logice will be here.
  } else {
    alert("Please enter only alphabets");
    return false;
  }

  var b = document.forms["myform"]["usrclg"].value;
  if (b == "") {
    alert("College must be filled out");
    return false;
  }
  var letters = /^[A-Za-z]+$/;
  if (b.test(letters)) {
    //Your logice will be here.
  } else {
    alert("Please enter only alphabets");
    return false;
  }

  row.cells[1].innerHTML = x;
  row.cells[2].innerHTML = y;
  row.cells[3].innerHTML = z;
  row.cells[4].innerHTML = a;
  row.cells[5].innerHTML = b;
}

<html>

<head>
</head>

<body>
  <form name="myform">
    <h1>
      <center> Admission form </center>
    </h1>
    <center>
      <tr>
        <td>Name :</td>
        <td><input type="text" name="usrname" PlaceHolder="Enter Your First Name" required></td>
      </tr>

      <tr>
        <td> Email ID :</td>
        <td><input type="text" name="usremail" PlaceHolder="Enter Your email address" pattern="[^@]+@[a-zA-Z]+\.[a-zA-Z]{2,6}" required></td>
      </tr>

      <tr>
        <td>Age :</td>
        <td><input type="number" name="usrage" PlaceHolder="Enter Your Age" required></td>
      </tr>

      <tr>
        <td>Dept :</td>
        <td><input type="text" name="usrdpt" PlaceHolder="Enter Dept"></td>
      </tr>

      <tr>
        <td>College :</td>
        <td><input type="text" name="usrclg" PlaceHolder="Enter college"></td>
      </tr>
    </center>

    <center>
      <br>
      <br>
      <tr>
        <td>
          <Button type="button" onclick="validateform()" id="Add">Add</button>
        </td>
        <td>
          <Button type="button" onclick="onUpdatefunction()" style="display:none;" id="update">update</button>
        </td>
        <td><button type="reset">Reset</button></td>
      </tr>
    </center>
    <br><br>
    <center>
      <table id="mytable" border="1">
        <tr>
          <th>SNO</th>
          <th>Name</th>
          <th>Email ID</th>
          <th>Age</th>
          <th>Dept</th>
          <th>College</th>
        </tr>
    </center>
    </table>
  </form>
</body>

</html>