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Inline formatting for a list, e.g., collaborative rich text


This library complements list-positions. It lets you add inline formatting to any list from that library: bold, font size, hyperlinks, etc. Inline formatting is a key component of rich text, and you can also use it for spreadsheet rows/columns and other lists.

Each formatting mark is defined in terms of Positions from the list-positions library, independent of a specific list or text string. So you can store formatting marks separately from the list itself, or share marks between multiple lists. In particular, you can share marks between lists on different devices, enabling DIY collaborative rich-text editing.

Example Use Cases

  1. Standard collaborative rich-text editing.
  2. Store annotations on a text document, in the style of atJSON but with the array indices replaced by immutable anchors that automatically accommodate future edits.
  3. In a rich-text editor with suggested changes, you could store suggested formatting marks separately and use Formatting.addMark/deleteMark to toggle between the current vs suggested formatting.
  4. Server authority: A user can optimistically add a formatting mark to their local instance, send it to a server, and then delete the mark if the server rejects it (e.g., it was outside the range that the user is allowed to edit).


list-formatting implements the core of the Peritext rich-text CRDT (though it is not itself a CRDT). It uses many of Peritext's concepts, which the previous link describes in detail; here is a self-contained summary.


An anchor is a spot in a list that is immediately before or after a position. Concretely, it is a JSON object of type Anchor:

import { Position } from "list-positions";

type Anchor = {
  pos: Position;
   * True for a "before" anchor, false for an "after" anchor.
  before: boolean;


A mark is an instruction to change the format of a range of values. For example, a mark may make a sentence bold, or add a hyperlink to a word.

Each mark starts and ends at an anchor. It sets a single key-value pair for all Positions between those anchors. You can visualize a mark as follows:

Anchors and an example mark

Concretely, a mark implements the interface IMark:

interface IMark {
  start: Anchor;
  end: Anchor;
  /** The format key, e.g., "bold" or "url". */
  key: string;
  /** The value at key, e.g., true or "". */
  value: any;

From Marks to Formatting

A given list can have many marks, often overlapping with each other. The current set of marks determines the list's current format.

The general principles are:

  1. Each mark affects all positions between its start and end anchors.
  2. Overlapping marks for different keys don't interact with each other. Instead, they combine to give multi-key formats like { bold: true, "font-size": 12 }.
  3. Overlapping marks for the same key are sorted in some way, e.g., using a timestamp (see TimestampFormatting below). The greatest mark under this sort order is the "winner" and determines the current value.

The sort order is how you override an existing mark, e.g., changing the font size from 12 to 16: you create a new mark that "wins" over the existing mark.

Formally, given the current set of marks, the current format at a position pos is an object of type Record<string, any>, given by:

  • For each format key key, find the greatest mark such that mark.key = key and the mark covers pos (mark.start < pos < mark.end).
  • If mark.value is not null, then add the entry { key: mark.value } to the format object. Otherwise, key is not present in the object.

The null-value rule lets you delete a format key: e.g., to change a range's format from { bold: true } to {} (unbolding), add a new mark with { key: "bold", value: null }.


Class Formatting

Class Formatting<M extends IMark> implements the above marks-to-formatting procedure. It is a local data structure storing a set of marks. Mutate the set using addMark(mark) and deleteMark(mark). Other methods let you query the formatting that results from the current set of marks:

  • getFormat(pos) returns the current format object at a Position.
  • formattedSlices(list) returns an efficient representation of the current formatting projected onto a specific list list. Specifically, it returns an array of slices in list order, where each slice is a list range { startIndex: number, endIndex: number } with a single format.
  • formattedSpans() returns an efficient representation of the current formatting, independent of a specific list. Specifically, it returns an array of spans in list order, where each span is a range { start: Anchor, end: Anchor } with a single format.

Class Formatting does not specify the sort order on marks. Instead, you choose the sort order, by extending the IMark interface with extra fields (type parameter M) and supplying a compareMarks function that uses those fields. Alternatively, you can use the TimestampFormatting class, which chooses a reasonable default sort order.


  • addMark and deleteMark return changes to the current formatting.
  • save() and load(savedState) save and load the current set of marks, similar to list-positions's save and load methods.
  • getActiveMarks(pos) and getMarks(pos) give you more info about the marks covering a given Position.
  • There is no way to modify an existing mark, and you should not mutate IMark objects in-place. Instead, delete the current mark and add a modified version.
  • For technical reasons, you cannot use Formatting with a list that contains MIN_POSITION or MAX_POSITION. Calling getFormat on such a Position will throw an error, as will using the "out-of-bounds" Anchors { pos: MIN_POSITION, before: true } or { pos: MAX_POSITION, before: false }.

Warning: Similar to list-positions's List class, you must manage metadata for a Formatting instance. Typically, you're already managing metadata for a List/Text/Outline/AbsList storing your actual values; it is then sufficient to share that list's Order with your Formatting instance, via the order constructor argument.

Class TimestampFormatting

Subclass of Formatting that chooses a reasonable default sort order.

TimestampFormatting uses marks of type TimestampMark, which is a JSON object:

type TimestampMark = {
  start: Anchor;
  end: Anchor;
  key: string;
  value: any;
  // TimestampMark metadata fields:
  creatorID: string;
  timestamp: number;

To create a TimestampMark, use TimestampFormatting.newMark.

TimestampFormatting's sort order uses Lamport timestamps, with ties broken by creatorID. This sort order works well in general, including in collaborative settings with or without a central server.

Class RichText

Convenience wrapper for Text with TimestampFormatting.

RichText has an API similar to a traditional rich-text data structure, combining indexed access, values, and formatting in a single object. E.g., it has a getFormatAt(index) method.

Notable methods:

  • insertWithFormat(index, format, ...values): Inserts values and applies new formatting marks as needed so that the values have the exact given format. This is a common operation when working with a rich-text editor: the editor tells you to insert some new values and what format they should have.
  • format(startIndex, endIndex, key, value, expand?): Formats the slice from startIndex to endIndex so that the given format key maps to value, by adding a new mark.
  • formattedChars(): Returns an efficient representation of the text's characters and their current formatting. It is similar to Quill's Delta format.

For other operations, you act on the Text or TimestampFormatting directly. E.g., to delete a value, call richText.text.deleteAt(index); to add a mark received from a collaborator, call richText.formatting.addMark(mark).

RichText's save() and load(savedState) methods save the Text, TimestampFormatting, and Order (metadata) states in a single JSON object. You can also save and load them separately.

If you don't want to use RichText (e.g., because you are using an List or Outline instead of a Text), you can access the same functionality using the Utilities below. Consider reading the RichText source code.


  • diffFormats(current, target): Returns changes (including null for deletions) to turn the current format into target. Core logic behind RichText.insertWithFormat.
  • sliceFromSpan, spanFromSlice: Convert between list-independent spans { start: Anchor, end: Anchor } and list-specific slices { startIndex: number, endIndex: number }.
  • Anchors static object: MIN_ANCHOR and MAX_ANCHOR; equals, compare, and validate functions for Anchors; indexOfAnchor and anchorAt.


Benchmarks are to-do. However, this library's implementation is similar to Collabs's CRichText and should have similar speed and metadata overhead. Cloud benchmarks showed that Collabs could handle 100+ simultaneous active users in a collaborative rich-text editor with frequent formatting operations, exceeding Google Docs' scalability.

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