The loading event fires when the document begins loading fonts.

Use the event name in methods like addEventListener(), or set an event handler property.

addEventListener("loading", (event) => {});

onloading = (event) => {};

In the following example, when the font Ephesis starts to load, "Font is loading…" is printed to the console.

document.fonts.onloading = () => {
  console.log("Font is loading");
};

(async () => {
  await document.fonts.load("16px Ephesis");
})();