@alcalzone/ansi-tokenize

@alcalzone/ansi-tokenize

Efficiently modify strings containing ANSI escape codes

If you find yourself modifying styled strings repeatedly, alternatives like slice-ansi may end up doing a lot of unnecessary work by re-parsing the string each time. This module provides a way to parse the string into an array of tokens (characters or ANSI codes), which can then be modified and re-serialized into a styled string.

Install

$ npm install @alcalzone/ansi-tokenize

Usage

Tokenize a string

import { tokenize } from "@alcalzone/ansi-tokenize";

// red "foo", followed by unstyled "bar"
const str = "\x1B[31mfoo\x1B[39mbar";
const tokens = tokenize(str);

// tokens will now look like this:
[
	{
		type: "ansi",
		code: "\x1B[31m",
		endCode: "\x1B[39m",
	},
	{
		type: "char",
		value: "f",
		fullWidth: false,
	},
	{
		type: "char",
		value: "o",
		fullWidth: false,
	},
	{
		type: "char",
		value: "o",
		fullWidth: false,
	},
	{
		type: "ansi",
		code: "\x1B[39m",
		endCode: "\x1B[39m",
	},
	{
		type: "char",
		value: "b",
		fullWidth: false,
	},
	{
		type: "char",
		value: "a",
		fullWidth: false,
	},
	{
		type: "char",
		value: "r",
		fullWidth: false,
	},
];

Each token is either a character

export interface Char {
	type: "char";
	value: string;
	fullWidth: boolean;
}

where

• value is the string representation of the character
• fullWidth is true if the character is full width (takes up 2 characters in monospace, like CJK characters)

or an ANSI code

export interface AnsiCode {
	type: "ansi";
	code: string;
	endCode: string;
}

where

• code is the ANSI code that starts the style
• and endCode is the corresponding ANSI code that ends the style.

An AnsiCode can also be an end code, in which case code and endCode will be the same.

Convert an array of tokens into an array of "styled" chars

This representation is a 1:1 mapping of the original string, but not very useful for modifying the string. The styledCharsFromTokens function converts a token array to an array of characters, where each character has an all currently active styles associated with it:

export interface StyledChar {
	type: "char";
	value: string;
	fullWidth: boolean;
	styles: AnsiCode[];
}

Using the above example:

import { tokenize, styledCharsFromTokens } from "@alcalzone/ansi-tokenize";

// red "foo", followed by unstyled "bar"
const str = "\x1B[31mfoo\x1B[39mbar";
const tokens = tokenize(str);

const styledChars = styledCharsFromTokens(tokens);

// styledChars will contain the following:
[
	{
		type: "char",
		value: "f",
		fullWidth: false,
		styles: [
			{
				type: "ansi",
				code: "\x1B[31m",
				endCode: "\x1B[39m",
			},
		],
	},
	{
		type: "char",
		value: "o",
		fullWidth: false,
		styles: [
			{
				type: "ansi",
				code: "\x1B[31m",
				endCode: "\x1B[39m",
			},
		],
	},
	{
		type: "char",
		value: "o",
		fullWidth: false,
		styles: [
			{
				type: "ansi",
				code: "\x1B[31m",
				endCode: "\x1B[39m",
			},
		],
	},
	{
		type: "char",
		value: "b",
		fullWidth: false,
		styles: [],
	},
	{
		type: "char",
		value: "a",
		fullWidth: false,
		styles: [],
	},
	{
		type: "char",
		value: "r",
		fullWidth: false,
		styles: [],
	},
];

Modify an array of styled characters

For modification simply edit the items in the array as necessary, as long as the following rules are followed:

• The code and endCode properties must match. You can use the ansi-styles module to do this.
• The fullWidth property must be correct. You can use the is-fullwidth-code-point module to do this, or if working with multiple strings, turn those into styled char arrays first.

E.g. to make the first o blue and bold:

import ansiStyles from "ansi-styles";

// ... include the above code

styledChars[1].styles = [
	{
		type: "ansi",
		code: ansiStyles.blue.open,
		endCode: ansiStyles.blue.close,
	},
	{
		type: "ansi",
		code: ansiStyles.bold.open,
		endCode: ansiStyles.bold.close,
	},
];

Serialize a styled character array back to a string

The styledCharsToString function converts a styled character array back to a string:

import { styledCharsToString } from "@alcalzone/ansi-tokenize";

// ... include the above code

const strOut = styledCharsToString(styledChars);

// str will now be '\x1B[31mf\x1B[34m\x1B[1mo\x1B[22m\x1B[31mo\x1B[39mbar'

This automatically figures out the least amount of escape codes necessary to achieve the desired result, as long as the styles arrays contain no unnecessary styles, e.g. blue + red foreground.

Changelog

0.2.1 (2025-10-20)

• Fix: Prevent dim and bold modifiers from canceling each other (#37)

0.2.0 (2025-04-24)

• Breaking: Require Node.js 18+
• Fix: Detect emojis as being full width

0.1.3 (2023-09-07)

• Fix: Support links

0.1.2 (2023-08-07)

• Fix: Reduce minimum Node.js version to 14.13.1

0.1.1 (2023-04-05)

• Fix: Active styles are now correctly reset at the end of the string

0.1.0 (2023-03-20)

Initial release