textarea with syntax highlighting

function inputHandler(event) {
    let code, inp;
    // Clone text into pre immediately
    // code = event.target.previousElementSibling.firstChild;
    // if (!code) return;
    inp = document.querySelector('#stack-input');
    code = inp.previousElementSibling.firstChild;
    code.textContent = inp.value
    // Highlight the syntax
    Prism.highlightElement(code); // , Prism.languages.javascript, 'javascript');
}
document.getElementById('stack-input').addEventListener('input', inputHandler);
document.getElementById('stack-input').addEventListener('focus', inputHandler);
window.addEventListener('load', inputHandler);