Range: startOffset property

The Range.startOffset read-only property returns a number representing where in the startContainer the Range starts.

If the startContainer is a Node of type Text, Comment, or CDATASection, then the offset is the number of characters from the start of the startContainer to the boundary point of the Range. For other Node types, the startOffset is the number of child nodes between the start of the startContainer and the boundary point of the Range.

To change the startOffset of a Range, use the Range.setStart method.

const range = document.createRange();
range.setStart(startNode, startOffset);
range.setEnd(endNode, endOffset);
const startRangeOffset = range.startOffset;