jQuery Get value of previous characters up till certain character within textarea

I making a text editor using jquery and electron. I would like to be able to make the text editor autocomplete HTML tags.

What I would like is when the user types a tag, for example <tag> a closing </tag> is than inserted. I am planning to do this by detecting when > is typed and saving the characters up until < as a variable. I would then insert those characters between </ and >. However I am unsure how to do this.

I have the following code:

function setCaretPosition(elemId, caretPos) {
    var el = document.getElementById(elemId);
    el.value = el.value;
    if (el !== null) {
        if (el.createTextRange) {
            var range = el.createTextRange();
            range.move('character', caretPos);
            range.select();
            return true;
        } else {
            if (el.selectionStart || el.selectionStart === 0) {
                el.focus();
                el.setSelectionRange(caretPos, caretPos);
                return true;
            } else {
                el.focus();
                return false;
            }
        }
    }
}

$("#input").on("keydown", function(e) {
    if (e.key === ">") {
        $("#input").focus();
        document.execCommand('insertHTML', true, '></>');
        var cursorPosition = $('#input').prop("selectionStart");
        setCaretPosition('input', cursorPosition - 3);

    }
});

and

<textarea class="form-control" id="input" spellcheck="false" wrap="off" placeholder="Get Creative!"></textarea>

Is there a way to do this?