@typedin/music-utilities/dist/cycle-of-fifths/index.js

A set of resources and modules I use to develop music applications.

import { AlterationEnum, DiatonicNoteEnum } from "../constants.js";
import { MinorThird } from "../interval-builders/Third.js";
import { PerfectFifth } from "../interval-builders/Fifth.js";
export function cycleOfFifths({ name, alteration }) {
    // Default key is not necessarily
    const INDEX_FOR_DEFAULT_KEY = 5;
    const cycle = new Array(12);
    // the idea here is to begin from C
    // put it in the middle
    // work downwards for flats
    // upwards for sharps
    // begin with the middle
    cycle[INDEX_FOR_DEFAULT_KEY] = {
        major: { name, alteration, octave: 4 },
        minor: {
            ...MinorThird({ name, alteration, octave: 4 }, "down"),
            octave: 4,
        },
    };
    // work our way backward towards flats
    for (let i = INDEX_FOR_DEFAULT_KEY; i >= 0; i--) {
        cycle[i - 1] = {
            major: { ...PerfectFifth(cycle[i].major, "down"), octave: 4 },
            minor: { ...PerfectFifth(cycle[i].minor, "down"), octave: 4 },
        };
    }
    // work our way upward towards sharps
    for (let i = INDEX_FOR_DEFAULT_KEY; i < 11; i++) {
        cycle[i + 1] = {
            major: { ...PerfectFifth(cycle[i].major, "up"), octave: 4 },
            minor: { ...PerfectFifth(cycle[i].minor, "up"), octave: 4 },
        };
    }
    let currentIndex = INDEX_FOR_DEFAULT_KEY;
    return {
        getSequence: () => cycle,
        next: () => cycle.at((currentIndex += 1)),
        previous: () => cycle.at((currentIndex -= 1)),
    };
}
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