jsmidgen (pronounced jay-smidgen or jer-smidgen) is a library that can be used to generate MIDI files in JavaScript. It currently provides output as a string, but there are plans to provide multiple output formats, including base64 and data URI.

The MIDI file structure is made up of one or more tracks, which contain one or more events. These events can be note on/off events, instrument changes, tempo changes, or more exotic things. A basic example is shown below:

var fs = require('fs');
var Midi = require('jsmidgen');

var file = new Midi.File();
var track = new Midi.Track();
file.addTrack(track);

track.addNote(0, 'c4', 64);
track.addNote(0, 'd4', 64);
track.addNote(0, 'e4', 64);
track.addNote(0, 'f4', 64);
track.addNote(0, 'g4', 64);
track.addNote(0, 'a4', 64);
track.addNote(0, 'b4', 64);
track.addNote(0, 'c5', 64);

fs.writeFileSync('test.mid', file.toBytes(), 'binary');

This example will create a MIDI file that will play an ascending C major scale, starting at middle C.

This library also has rudimentary support for a fluid (chained) style:

file = new Midi.File();
file
  .addTrack()

    .note(0, 'c4', 32)
    .note(0, 'd4', 32)
    .note(0, 'e4', 32)
    .note(0, 'f4', 32)
    .note(0, 'g4', 32)
    .note(0, 'a4', 32)
    .note(0, 'b4', 32)
    .note(0, 'c5', 32)

    // church organ
    .instrument(0, 0x13)

    // by skipping the third arguments, we create a chord (C major)
    .noteOn(0, 'c4', 64)
    .noteOn(0, 'e4')
    .noteOn(0, 'g4')

    // by skipping the third arguments again, we stop all notes at once
    .noteOff(0, 'c4', 47)
    .noteOff(0, 'e4')
    .noteOff(0, 'g4')

    //alternatively, a chord may be created with the addChord function
    .addChord(0, ['c4', 'e4', 'g4'], 64)

    .noteOn(0, 'c4', 1)
    .noteOn(0, 'e4')
    .noteOn(0, 'g4')
    .noteOff(0, 'c4', 384)
    .noteOff(0, 'e4')
    .noteOff(0, 'g4')
    ;

fs.writeFileSync('test2.mid', file.toBytes(), 'binary');

Note the use of setInstrument() to change to a church organ midway through, and the use of noteOn()/noteOff() to produce chords.

• addTrack() - Add a new Track object to the file and return the new track.
• addTrack(track) - Add the given Track object to the file and return the file.
• toBytes() - Serialize the file to an array of bytes.
• toUint8Array() - Serialize the file to a typed array of bytes (Uint8Array).
• toBlob([genericType]) - Serialize the file to a Blob object. If genericType is true, the blob will have the generic application/octet-stream MIME type, otherwise it will be the standard audio/x-midi MIME type.

Time and duration are specified in "ticks", and there is a hardcoded value of 128 ticks per beat. This means that a quarter note has a duration of 128.

Pitch can be specified by note name with octave (a#4) or by note number (60). Middle C is represented as c4 or 60.

• addNote(channel, pitch, duration[, time[, velocity]])

  Add a new note with the given channel, pitch, and duration

  • If time is given, delay that many ticks before starting the note. This can be used for adding rests.
  • If velocity is given, strike the note with that velocity.

• addNoteOn(channel, pitch[, time[, velocity]])

  Start a new note with the given channel and pitch

  • If time is given, delay that many ticks before starting the note. This can be used for adding rests.
  • If velocity is given, strike the note with that velocity.

• addNoteOff(channel, pitch[, time[, velocity]])

  End a note with the given channel and pitch

  • If time is given, delay that many ticks before ending the note.
  • If velocity is given, strike the note with that velocity.

• addChord(channel, chord[, velocity])

  Add a chord with the given channel and pitches

  • Accepts an array of pitches to play as a chord.
  • If velocity is given, strike the chord with that velocity.

• setInstrument(channel, instrument[, time]).

  Change the given channel to the given instrument

  • If time is given, delay that many ticks before making the change.

• setTempo(bpm[, time])

  Set the tempo to bpm beats per minute

  • If time is given, delay that many ticks before making the change.

• setTimeSignature(numerator, denominator[, time])

  Set the time signature

  • For example, 3/4 time would have numerator 3 and denominator 4.
  • The denominator must be a power of 2 (e.g. 1, 2, 4, 8, 16, 32).
  • If time is given, delay that many ticks before making the change.

• setKeySignature(accidentals[, minor[, time]])

  Set the key signature

  • If accidentals is positive, it is the number of sharps in the key signature.
  • If accidentals is negative, it is the number of flats in the key signature.
  • If minor is true, the key signature is a minor key.
  • If time is given, delay that many ticks before making the change.