The add() method of the Highlight interface adds a new Range object to a highlight, to be styled using the CSS Custom Highlight API.

Highlight is a Set-like object, so this is similar to using Set.add().

range

A Range object to add to the Highlight.

The Highlight object, with the added range.

The code snippet below shows how to add two ranges to a new highlight object:

const highlight = new Highlight();

const range1 = new Range();
const range2 = new Range();

highlight.add(range1).add(range2);

console.log(highlight.size); // 2

• The CSS Custom Highlight API
• CSS Custom Highlight API: The Future of Highlighting Text Ranges on the Web