TextFormat: underlineThickness property

Limited availability

This feature is not Baseline because it does not work in some of the most widely-used browsers.

Experimental: This is an experimental technology
Check the Browser compatibility table carefully before using this in production.

The underlineThickness property of the TextFormat interface indicates the thickness of the underline that needs to be applied to the text range that is being formatted.


A String that is one of the following values:

  • "none": No underline.
  • "thin": A thin underline.
  • "thick": A thick underline.


Reading the underline thickness that needs to be applied

The following example shows how to use the textformatupdate event's underlineThickness property to determine the underline thickness that needs to be applied to the text being formatted. Note that the event listener callback in this example is only called when using an IME window to compose text.

<div id="editor" style="height:200px;background:#eee;"></div>
const editorEl = document.getElementById("editor");
const editContext = new EditContext(editorEl);
editorEl.editContext = editContext;

editContext.addEventListener("textformatupdate", (e) => {
  const formats = e.getTextFormats();

  for (const format of formats) {
      `IME wants to apply a ${format.underlineThickness} underline between ${format.rangeStart} and ${format.rangeEnd}.`,


EditContext API
# dom-textformat-underlinethickness

Browser compatibility

BCD tables only load in the browser

See also