mso-conditional-parser

mso-conditional-parser

Parser and translator for MSO (Outlook) conditional comments in HTML email.

Parses all real-world MSO comment patterns, validates conditions against known Outlook versions and operators, and translates them into human-readable English.

Used as the core engine by eslint-plugin-mso-email and the MSO Conditional Comments VS Code extension.

Installation

npm install mso-conditional-parser

Requires Node.js 18+. ES module only ("type": "module").

API

parseMsoComment(str)

Parses an MSO conditional comment opener.

Returns a result object or null if the input is not a recognised MSO opener.

import { parseMsoComment } from 'mso-conditional-parser';

parseMsoComment('<!--[if gte mso 16]>');
// {
//   type: 'downlevel-hidden',
//   condition: 'gte mso 16',
//   translation: 'Outlook 2016, 2019, and 365 and newer',
//   isValid: true
// }

parseMsoComment('<!--[if mos]>');
// {
//   type: 'downlevel-hidden',
//   condition: 'mos',
//   translation: 'mos',
//   isValid: false,
//   error: "Use the keyword 'mso', not 'mos'",
//   conditionFix: 'mso'
// }

Supported opener patterns:

Pattern	Type
<!--[if mso]>	downlevel-hidden
<!--[if gte mso 16]>	downlevel-hidden
<!--[if !mso]><!--	downlevel-revealed
<![if !mso]>	downlevel-revealed (non-standard)

Result shape:

Field	Type	Description
type	string	'downlevel-hidden' or 'downlevel-revealed'
condition	string	Raw condition string extracted from the comment
translation	string	Human-readable English translation
isValid	boolean	true if the condition is syntactically valid
error	string	Present when isValid is false
conditionFix	string	Optional replacement condition when a safe autofix exists

validateCondition(condition)

Returns an error message for an invalid condition string, or null when valid.

import { validateCondition } from 'mso-conditional-parser';

validateCondition('mos'); // "Use the keyword 'mso', not 'mos'"
validateCondition('gte mso 16'); // null

getConditionFix(condition)

Returns a replacement condition string when a deterministic fix exists (for example mos → mso), or null otherwise.

import { getConditionFix } from 'mso-conditional-parser';

getConditionFix('mos'); // 'mso'
getConditionFix('gte mso 16'); // null

parseMsoEndComment(str)

Parses an MSO conditional comment closer.

Returns { type, isClosing: true } or null if not recognised.

import { parseMsoEndComment } from 'mso-conditional-parser';

parseMsoEndComment('<![endif]-->');
// { type: 'downlevel-hidden-end', isClosing: true }

parseMsoEndComment('<![endif]>');
// { type: 'downlevel-revealed-end', isClosing: true }

translateCondition(condition)

Translates a raw condition string into English without full parsing.

import { translateCondition } from 'mso-conditional-parser';

translateCondition('gte mso 16');  // 'Outlook 2016, 2019, and 365 and newer'
translateCondition('!mso');        // 'non-Outlook email clients'
translateCondition('mso 12');      // 'Outlook 2007'

isMsoComment(str)

Returns true if the string is any MSO comment — opener or closer.

import { isMsoComment } from 'mso-conditional-parser';

isMsoComment('<!--[if mso]>');   // true
isMsoComment('<![endif]-->');    // true
isMsoComment('<p>hello</p>');    // false

Supported conditions

Syntax	Example	Meaning
mso	<!--[if mso]>	Any Outlook version
!mso	<!--[if !mso]><!--	Non-Outlook clients
mso <version>	<!--[if mso 16]>	Exact Outlook version
<op> mso <version>	<!--[if gte mso 14]>	Version comparison
!mso <version>	<!--[if !mso 16]>	Not this version
IE / !IE	<!--[if IE]>	Legacy Internet Explorer conditions
<op> IE <version>	<!--[if gte IE 8]>	IE version comparison
Compound & / |	<!--[if (gt mso 9)&(lte mso 11)]>	AND / OR

Valid operators: gte, gt, lte, lt, eq

Valid Outlook versions: 9 (2000), 10 (2002), 11 (2003), 12 (2007), 14 (2010), 15 (2013), 16 (2016/2019/365)

Note: there is no MSO version 13 — Outlook skips from 12 (2007) to 14 (2010).

License

MIT