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



npm install libmime


var libmime = require('libmime');


NB! charset decoding methods (eg. decodeWords) throw if an unknown charset is used

Encoded Words


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


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

Becomes with UTF-8 and Quoted-printable encoding



Decodes a string from mime encoded word format.

libmime.decodeWord(str) → String
  • str - String to be decoded



will become

See on õhin test


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


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



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


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

results in

Content-Type: multipart/alternative;


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


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)



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


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


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


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


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


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


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: '...'}]


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


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


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


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


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