js

close()
close(returnValue)

html

<dialog id="dialog">
  <button type="button" id="close">Close</button>
  <button type="button" id="close-w-value">Close w/ return value</button>
</dialog>

<button id="open">Open dialog</button>

#log {
  height: 170px;
  overflow: scroll;
  padding: 0.5rem;
  border: 1px solid black;
}

const logElement = document.getElementById("log");
function log(text) {
  logElement.innerText = `${logElement.innerText}${text}\n`;
  logElement.scrollTop = logElement.scrollHeight;
}

js

const dialog = document.getElementById("dialog");
const openButton = document.getElementById("open");
const closeButton = document.getElementById("close");
const closeWithValueButton = document.getElementById("close-w-value");

// Update button opens a modal dialog
openButton.addEventListener("click", () => {
  // Reset the return value
  dialog.returnValue = "";
  // Show the dialog
  dialog.showModal();
});

// Close button closes the dialog box
closeButton.addEventListener("click", () => {
  dialog.close();
});

// Close button closes the dialog box with a return value
closeWithValueButton.addEventListener("click", () => {
  dialog.close(`Closed at ${new Date().toLocaleTimeString()}`);
});

// Form close button closes the dialog box
dialog.addEventListener("close", () => {
  log(`Dialog closed. Return value: "${dialog.returnValue}"`);
});

Note:

You know you can also automatically close a <dialog> by submitting a <form> element with a method="dialog" attribute.