Function keyFromFifths

keyFromFifths(fifths: number, mode?: KeyMode): ResolvedKey
The key a signature denotes: the inverse of keySignatureFifths for the two modes a signature is conventionally read in.

A signature names two keys — three sharps is both A major and F# minor — so the mode decides which one is built. The tonic is spelled the way that key is written, including the theoretical keys past ±7 (nine sharps is D# major).
Parameters
- fifths: number
  The signed number of sharps (positive) or flats (negative).
- mode: KeyMode = 'major'
  Which of the signature's two keys to build; defaults to major.
Returns ResolvedKey
The spelled tonic and the matching key/scale.
Throws
If fifths is not an integer in [-12, 12], or mode is not one of 'major' or 'minor'.
Example
```
import { keyFromFifths, formatNote } from '@libraz/libcantus';
formatNote(keyFromFifths(-2).tonic); // 'Bb'
formatNote(keyFromFifths(-2, 'minor').tonic); // 'G'
```
- Defined in src/theory/scale/signature.ts:173