JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" />

JavaScript

// Thanks http://blog.vishalon.net/index.php/javascript-getting-and-setting-caret-position-in-textarea/
function getCaretPosition(ctrl) {
    var CaretPos = 0;    // IE Support
    if (document.selection) {
        ctrl.focus();
        var Sel = document.selection.createRange();
        Sel.moveStart('character', -ctrl.value.length);
        CaretPos = Sel.text.length;
    }
    // Firefox support
    else if (ctrl.selectionStart || ctrl.selectionStart == '0') {
        CaretPos = ctrl.selectionStart;
    }
    
    return CaretPos;
}

function setCaretPosition(ctrl, pos) {
    if (ctrl.setSelectionRange) {
        ctrl.focus();
        ctrl.setSelectionRange(pos,pos);
    }
    else if (ctrl.createTextRange) {
        var range = ctrl.createTextRange();
        range.collapse(true);
        range.moveEnd('character', pos);
        range.moveStart('character', pos);
        range.select();
    }
}


// The real work

$(function() {
    $('input').keyup(function() {
        // Remember original caret position
        var caretPosition = getCaretPosition(this);
        
        // Uppercase-ize contents
        this.value = this.value.toUpperCase();
        
        // Reset caret position
        // (we ignore selection length, as typing deselects anyway)
        setCaretPosition(this, caretPosition);
    });
});