libmime

What is libmime?

The libmime npm package is a utility library for handling MIME types. It provides functionalities for parsing and formatting MIME types, which are essential for handling file types and content negotiation in web and email development. With libmime, developers can easily determine the MIME type of a file, create MIME-encoded words, and parse MIME-encoded strings.

What are libmime's main functionalities?

Determining MIME type from a filename

This feature allows you to get the MIME type of a file based on its extension. It's useful for setting Content-Type headers when serving files over HTTP.

const libmime = require('libmime');
const mimeType = libmime.getType('example.pdf');
console.log(mimeType); // 'application/pdf'

Getting a file extension from a MIME type

This functionality enables you to find the appropriate file extension for a given MIME type, which can be useful when generating files dynamically.

const libmime = require('libmime');
const extension = libmime.getExtension('application/pdf');
console.log(extension); // 'pdf'

Encoding and decoding MIME-encoded words

libmime can encode and decode MIME-encoded words, which is particularly useful for handling email headers that include characters outside the ASCII range.

const libmime = require('libmime');
const encodedWord = libmime.encodeWord('Hello äöü', 'Q');
const decodedWord = libmime.decodeWord(encodedWord);
console.log(encodedWord);
console.log(decodedWord);

Other packages similar to libmime

mime

The 'mime' package offers similar functionalities for determining MIME types and extensions. However, it focuses more on the mapping between file extensions and MIME types and does not include functionality for encoding and decoding MIME-encoded words.

mime-types

Similar to 'mime', the 'mime-types' package provides utility functions for working with MIME types and file extensions. It is a lightweight alternative, offering a simple API for MIME type lookup and extension retrieval without the encoding/decoding capabilities of libmime.

content-type

The 'content-type' package is designed to parse and format HTTP Content-Type headers. While it deals with MIME types as part of its functionality, its focus is narrower, targeting HTTP header manipulation rather than the broader scope of MIME type handling offered by libmime.

README

libmime

libmime provides useful MIME related functions. For Quoted-Printable and Base64 encoding and decoding see libqp and libbase64.

Installation

npm:

npm install libmime

Usage

var libmime = require('libmime');

Methods

Encoded Words

#encodeWord

Encodes a string into mime encoded word format.

libmime.encodeWord(str [, mimeWordEncoding[, maxLength]]) → String

• str - String or Buffer to be encoded
• mimeWordEncoding - Encoding for the mime word, either Q or B (default is 'Q')
• maxLength - If set, split mime words into several chunks if needed

Example

libmime.encodeWord('See on õhin test', 'Q');

Becomes with UTF-8 and Quoted-printable encoding

=?UTF-8?Q?See_on_=C3=B5hin_test?=

#decodeWord

Decodes a string from mime encoded word format.

libmime.decodeWord(str) → String

• str - String to be decoded

Example

libmime.decodeWord('=?UTF-8?Q?See_on_=C3=B5hin_test?=');

will become

See on õhin test

#encodeWords

Encodes non ascii sequences in a string to mime words.

libmime.encodeWords(str[, mimeWordEncoding[, maxLength]) → String

• str - String or Buffer to be encoded
• mimeWordEncoding - Encoding for the mime word, either Q or B (default is 'Q')
• maxLength - If set, split mime words into several chunks if needed

#decodeWords

Decodes a string that might include one or several mime words. If no mime words are found from the string, the original string is returned

libmime.decodeWords(str) → String

• str - String to be decoded

Folding

#foldLines

Folds a long line according to the RFC 5322. Mostly needed for folding header lines.

libmime.foldLines(str [, lineLength[, afterSpace]]) → String

• str - String to be folded
• lineLength - Maximum length of a line (defaults to 76)
• afterSpace - If true, leave a space in the end of a line

Example

libmime.foldLines('Content-Type: multipart/alternative; boundary="----zzzz----"')

results in

Content-Type: multipart/alternative;
     boundary="----zzzz----"

#encodeFlowed

Adds soft line breaks to content marked with format=flowed options to ensure that no line in the message is never longer than lineLength.

libmime.encodeFlowed(str [, lineLength]) → String

• str Plaintext string that requires wrapping
• lineLength (defaults to 76) Maximum length of a line

#decodeFlowed

Unwraps a plaintext string in format=flowed wrapping.

libmime.decodeFlowed(str [, delSp]) → String

• str Plaintext string with format=flowed to decode
• delSp If true, delete leading spaces (delsp=yes)

Headers

#decodeHeader

Unfolds a header line and splits it to key and value pair. The return value is in the form of {key: 'subject', value: 'test'}. The value is not mime word decoded, you need to do your own decoding based on the rules for the specific header key.

libmime.decodeHeader(headerLine) → Object

• headerLine - Single header line, might include linebreaks as well if folded

#decodeHeaders

Parses a block of header lines. Does not decode mime words as every header might have its own rules (eg. formatted email addresses and such).

Return value is an object of headers, where header keys are object keys and values are arrays.

libmime.decodeHeaders(headers) → Object

• headers - Headers string

#parseHeaderValue

Parses a header value with key=value arguments into a structured object. Useful when dealing with content-type and such. Continuation encoded params are joined into mime encoded words.

parseHeaderValue(valueString) → Object

• valueString - a header value without the key

Example

parseHeaderValue('content-type: text/plain; CHARSET="UTF-8"');

Outputs

{
    "value": "text/plain",
    "params": {
        "charset": "UTF-8"
    }
}

#buildHeaderValue

Joins structured header value together as 'value; param1=value1; param2=value2'

buildHeaderValue(structuredHeader) → String

• structuredHeader - a header value formatted with parseHeaderValue

filename argument is encoded with continuation encoding if needed

#buildHeaderParam

Encodes and splits a header param value according to RFC2231 Parameter Value Continuations.

libmime.buildHeaderParam(key, str, maxLength) → Array

• key - Parameter key (eg. filename)
• str - String or an Buffer value to encode
• maxLength - Maximum length of the encoded string part (not line length). Defaults to 50

The method returns an array of encoded parts with the following structure: [{key:'...', value: '...'}]

Example

libmime.buildHeaderParam('filename', 'filename õäöü.txt', 20);
→
[ { key: 'filename*0*', value: 'utf-8\'\'filename%20' },
  { key: 'filename*1*', value: '%C3%B5%C3%A4%C3%B6' },
  { key: 'filename*2*', value: '%C3%BC.txt' } ]

This can be combined into a properly formatted header:

Content-disposition: attachment; filename*0*=utf-8''filename%20
  filename*1*=%C3%B5%C3%A4%C3%B6; filename*2*=%C3%BC.txt

MIME Types

#detectExtension

Returns file extension for a content type string. If no suitable extensions are found, 'bin' is used as the default extension.

libmime.detectExtension(mimeType) → String

• mimeType - Content type to be checked for

Example

libmime.detectExtension('image/jpeg') // returns 'jpeg'

#detectMimeType

Returns content type for a file extension. If no suitable content types are found, 'application/octet-stream' is used as the default content type

libmime.detectMimeType(extension) → String

• extension Extension (or filename) to be checked for

Example

libmime.detectExtension('logo.jpg') // returns 'image/jpeg'

License

MIT

Changelog

v3.0.0 2016-12-08

• Updated encoded-word generation. Previously a minimal value was encoded, so it was possible to have multiple encoded words in a string separated by non encoded-words. This was an issue with some webmail clients that stripped out the non-encoded parts between encoded-words so the updated method uses wide match by encoding from the first word with unicode characters to the last word. "a =?b?= c =?d?= e" -> "a =?bcd?= e"