Function to get information around cursor

getCursorContext.js

/* @flow */

/* this code heavily borrows from https://github.com/zurb/tribute */

export type CursorContext = {
  textNode: Node;
  textNodeParent: Node;
  textNodeContent: string;
  textBeforeCursor: string;
  textAfterCursor: string;
  cursorCharacterPosition: number;
};

export default function getCursorContext(): ?CursorContext {
  let cursorTextNodeAndParent = getTextNodeAndParent();
  if(!cursorTextNodeAndParent) return null;

  // we now normalize the textNodeParent to make sure there's only one text node
  // makes subsequent code a lot simpler
  cursorTextNodeAndParent.textNodeParent.normalize();
  cursorTextNodeAndParent = getTextNodeAndParent();
  if(!cursorTextNodeAndParent) return null;

  const {textNode, textNodeParent, selection} = cursorTextNodeAndParent;

  const range = selection.getRangeAt(0);
  const textNodeContent = textNode.textContent;
  const cursorCharacterPosition = range.startOffset;
  if(!(cursorCharacterPosition >= 0)) return null;

  return {
    textNode, textNodeParent, textNodeContent, cursorCharacterPosition,
    textBeforeCursor: textNodeContent.substring(0, cursorCharacterPosition),
    textAfterCursor: textNodeContent.substr(cursorCharacterPosition)
  };
}

function getTextNodeAndParent(): ?{textNode: Node; textNodeParent: Node; selection: Selection;} {
  const selection = document.getSelection();
  if(!selection) return null;

  const textNode = selection.anchorNode;
  if(!textNode || textNode.nodeType !== Node.TEXT_NODE) return null;

  const textNodeParent = textNode.parentNode;
  if(!textNodeParent) return null;

  return {textNode, textNodeParent, selection};
}