How to make "Enter" key in a textarea submit a form

Try this (note that pressing Enter submits, and Shift+Enter adds a new line).

$("#textareaId").keypress(function (e) {
    if(e.which === 13 && !e.shiftKey) {
        e.preventDefault();
    
        $(this).closest("form").submit();
    }
});

Vanilla JavaScript solution

function submitOnEnter(event){
    if(event.which === 13){
        event.target.form.dispatchEvent(new Event("submit", {cancelable: true}));
        event.preventDefault(); // Prevents the addition of a new line in the text field (not needed in a lot of cases)
    }
}

document.getElementById("usermsg").addEventListener("keypress", submitOnEnter);

If you would prefer to not submit if Shift is clicked all you need to do is change line 2 to:

if(event.which === 13 && !event.shiftKey){

To clear the textarea just add this inside the if statement body

event.target.value = "";

To use this with Internet Explorer 11 change line 3 to:

var newEvent = document.createEvent("Event");
newEvent.initEvent("submit", false, true);
event.target.form.dispatchEvent(newEvent);

But wait? Why not use event.target.form.submit();?

When you invoke the submit method of the form the event listeners will not be called.

Demo:

function submitOnEnter(event){
    if(event.which === 13){
        event.target.form.dispatchEvent(new Event("submit", {cancelable: true}));
        event.preventDefault(); // Prevents the addition of a new line in the text field (not needed in a lot of cases)
    }
}

document.getElementById("usermsg").addEventListener("keypress", submitOnEnter);

document.getElementById("form").addEventListener("submit", (event) => {
    event.preventDefault();
    console.log("form submitted");
});

<form id="form">
    <textarea id="usermsg"></textarea>
    <button type="Submit">Submit</button>
</form>

I have created a jsfiddle with an example on how to do it with jQuery and the keypressfunction and which property: http://jsfiddle.net/GcdUE/

Not sure exactly what you are asking for more than this, so please specify your question further if possible.

$(function() {
    $("#usermsg").keypress(function (e) {
        if(e.which == 13) {
            //submit form via ajax, this is not JS but server side scripting so not showing here
            $("#chatbox").append($(this).val() + "<br/>");
            $(this).val("");
            e.preventDefault();
        }
    });
});

Well, supposing you were using jquery it would be a simple listener on your input field:

In your footer before </body>:

<script type="text/javascript">
$(document).ready(function(){
    $('#usermsg').keypress(function(e){
      if(e.which == 13){
           // submit via ajax or
           $('form').submit();
       }
    });
});
</script>

In your html head add this:

<script src="https://ajax.googleapis.com/ajax/libs/jqueryui/1/jquery-ui.min.js" type="text/javascript" charset="utf-8"></script> 

but if js or jquery is out of the question then perhaps update your question to specifically exclude it.