note-transposer/index.js

Music transposition made easy

var parse = require('music-notation/pitch/parse')
var str = require('music-notation/pitch/str')
var operation = require('music-notation/operation')(parse, str)

/**
 * Transposes a note by an interval.
 *
 * Given a note and an interval it returns the transposed note. It can be used
 * to add intervals if both parameters are intervals.
 *
 * The order of the parameters is indifferent.
 *
 * This function is currified so it can be used to map arrays of notes.
 *
 * @name transpose
 * @function
 * @param {String|Array} interval - the interval. If its false, the note is not
 * transposed.
 * @param {String|Array} note - the note to transpose
 * @return {String|Array} the note transposed
 *
 * @example
 * var transpose = require('note-transposer')
 * transpose('3m', 'C4') // => 'Eb4'
 * transpose('C4', '3m') // => 'Eb4'
 * tranpose([1, 0, 2], [3, -1, 0]) // => [3, 0, 2]
 * ['C', 'D', 'E'].map(transpose('3M')) // => ['E', 'F#', 'G#']
 */
var transpose = operation(function (i, n) {
  if (i === false) return n
  else if (!Array.isArray(i) || !Array.isArray(n)) return null
  else if (i.length === 1 || n.length === 1) return [n[0] + i[0]]
  var d = i.length === 2 && n.length === 2 ? null : n[2] || i[2]
  return [n[0] + i[0], n[1] + i[1], d]
})

if (typeof module === 'object' && module.exports) module.exports = transpose
if (typeof window !== 'undefined') window.transpose = transpose