So, for applying alphabet only validation in js we will use onkeypress event of input boxes. So you can check the below code for applying the validation.

Below is a code in HTML and JavaScript to alphabet validate a text field

<form>
    <div class="form-group row">
        <label for="inputPassword" class="col-sm-2 col-form-label">Enter Only Alphabets</label>
        <div class="col-sm-10">
            <input type="text" class="form-control" id="inputAlphabet" placeholder="Enter Only Alphabets" value="" name="inputAlphabet" onkeypress="return onlyAlphabets(event)" />
        </div>
    </div>
</form>

Here is how to validate the input only to accept alphabet this will only take alphabet values like "abcc". We are checking event keycodes for checking if the entered character is a valid one.

<script>
    function onlyAlphabets(evt) {
        try {
            evt = (evt) ? evt : window.event;
            var charCode = (evt.which) ? evt.which : evt.keyCode;
            if ((charCode > 64 && charCode < 91) || (charCode > 96 && charCode < 123))
                return true;
            else
                return false;
        }
        catch (err) {
            alert(err.Description);
        }
    }
</script>

You can copy complete code from here .

<form>
    <div class="form-group row">
        <label for="inputPassword" class="col-sm-2 col-form-label">Enter Only Alphabets</label>
        <div class="col-sm-10">
            <input type="text" class="form-control" id="inputAlphabet" placeholder="Enter Only Alphabets" value="" name="inputAlphabet" onkeypress="return onlyAlphabets(event)" />
        </div>
    </div>
</form>
<script>
    function onlyAlphabets(evt) {
        try {
            evt = (evt) ? evt : window.event;
            var charCode = (evt.which) ? evt.which : evt.keyCode;
            if ((charCode > 64 && charCode < 91) || (charCode > 96 && charCode < 123))
                return true;
            else
                return false;
        }
        catch (err) {
            alert(err.Description);
        }
    }
</script>

So, this is how you can apply alphabet only validation using javascript.