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JSMime is a MIME parsing and composition library that is written completely in JavaScript using ES6 functionality and WebAPIs (where such APIs exist). There are a few features for which a standardized WebAPI does not exist; for these, external JavaScript libraries are used.

The MIME parser consists of three logical phases of translation:

  1. Build the MIME (and pseudo-MIME) tree.
  2. Convert the MIME tree into a list of body parts and attachments.
  3. Use the result to drive a displayed version of the message.

The first stage is located in mimeparser.js. The latter stages have yet to be implemented.

Dependencies

This code depends on the following ES6 features and Web APIs:

  • ES6 generators
  • ES6 Map and Set
  • ES6 @@iterator support (especially for Map and Set)
  • ES6 let
  • ES6 let-destructuring
  • ES6 const
  • Typed arrays (predominantly Uint8Array)
  • btoa, atob (found on global Windows or WorkerScopes)
  • TextDecoder

Versions and API stability

As APIs require some use and experimentation to get a feel for what works best, the APIs may change between successive version updates as uses indicate substandard or error-prone APIs. Therefore, there will be no guarantee of API stability until version 1.0 is released.

This code is being initially developed as an effort to replace the MIME library within Thunderbird. New versions will be released as needed to bring new support into the Thunderbird codebase; version 1.0 will correspond to the version where feature-parity with the old MIME library is reached. The set of features which will be added before 1.0 are the following:

  • S/MIME encryption and decryption
  • PGP encryption and decryption
  • IMAP parts-on-demand support
  • Support for text/plain to HTML conversion for display
  • Support for HTML downgrading and sanitization for display
  • Support for all major multipart types
  • Ability to convert HTML documents to text/plain and multipart/related
  • Support for building outgoing messages
  • Support for IDN and EAI
  • yEnc and uuencode decoding support
  • Support for date and Message-ID/References-like headers

Other features than these may be added before version 1.0 is released (most notably TNEF decoding support), but they are not considered necessary to release a version 1.0.