JSFiddle - React, Tailwind, and code Playground

HTML

<form name="thisForm">
<textarea id="text" name="text" rows="6" cols="40">
keep this row lowercase
cApItAliZe thIS rOW
KEEP THIS UPPERCASE
</textarea>
</form>

<a href="javascript:void();" id="link">Capitalize selected text</a>

JavaScript

function getInputSelection(el) {
    var start = 0, end = 0, normalizedValue, range,
        textInputRange, len, endRange;

    if (typeof el.selectionStart == "number" && typeof el.selectionEnd == "number") {
        start = el.selectionStart;
        end = el.selectionEnd;
    } else {
        range = document.selection.createRange();

        if (range && range.parentElement() == el) {
            len = el.value.length;
            normalizedValue = el.value.replace(/\r\n/g, "\n");

            // Create a working TextRange that lives only in the input
            textInputRange = el.createTextRange();
            textInputRange.moveToBookmark(range.getBookmark());

            // Check if the start and end of the selection are at the very end
            // of the input, since moveStart/moveEnd doesn't return what we want
            // in those cases
            endRange = el.createTextRange();
            endRange.collapse(false);

            if (textInputRange.compareEndPoints("StartToEnd", endRange) > -1) {
                start = end = len;
            } else {
                start = -textInputRange.moveStart("character", -len);
                start += normalizedValue.slice(0, start).split("\n").length - 1;

                if (textInputRange.compareEndPoints("EndToEnd", endRange) > -1) {
                    end = len;
                } else {
                    end = -textInputRange.moveEnd("character", -len);
                    end += normalizedValue.slice(0, end).split("\n").length - 1;
                }
            }
        }
    }

    return {
        start: start,
        end: end
    };
}

function capitalizeSelectedText(el) {
    var sel       = getInputSelection(el),
        val       = el.value,
        selection = val.slice(0, sel.start) + val.slice(sel.end);
    el.value = selection.toLowerCase();
}

$("#link").click(function() {
    var el = document.getElementById("text");
    capitalizeSelectedText(el);
});