Toggle Summary Message. Changes the content of a details' summary to indicate whether clicking it will open or close it. Assumes the <summary> lies directly under the <details> tag and contains `data-message-open` and `data-message-close` attributes with the respective values for each case. View demo: https://codepen.io/cballenar/pen/PoagMxz

toggleSummaryMessage.js

/**
 * Toggle Summary Message.
 * Changes the content of a details' summary to indicate whether clicking it
 * will open or close it. Assumes the <summary> lies directly under the
 * <details> tag and contains `data-message-open` and `data-message-close`
 * attributes with the respective values for each case.
 *
 * @param {Event} event
 */
function toggleSummaryMessage(event) {
    const summary = event.target;
    summary.innerText = summary.parentElement.open ? summary.dataset.messageOpen : summary.dataset.messageClose;
}
// call for all summaries
const summaries = document.querySelectorAll('details summary');
summaries.forEach((detail)=>detail.addEventListener('click',toggleSummaryMessage));