feat: adding ability to get the frequency at the FFT index

`getFrequencyOfIndex`

Tone/component/analysis/FFT.test.ts

@@ -35,6 +35,13 @@ describe("FFT", () => {
 		fft.dispose();
 	});
 
+	it("can get the frequency values of each index of the return array", () => {
+		const fft = new FFT(32);
+		expect(fft.getFrequencyOfIndex(0)).to.be.closeTo(0, 1);
+		expect(fft.getFrequencyOfIndex(16)).to.be.closeTo(fft.context.sampleRate / 4, 1);
+		fft.dispose();
+	});
+
 	it("can run waveform analysis", (done) => {
 		const noise = new Noise();
 		const fft = new FFT(256);

Tone/component/analysis/FFT.ts

@@ -1,8 +1,9 @@
 import { ToneAudioNode } from "../../core/context/ToneAudioNode";
 import { dbToGain } from "../../core/type/Conversions";
-import { NormalRange, PowerOfTwo } from "../../core/type/Units";
+import { Hertz, NormalRange, PowerOfTwo } from "../../core/type/Units";
 import { optionsFromArguments } from "../../core/util/Defaults";
 import { MeterBase, MeterBaseOptions } from "./MeterBase";
+import { assert } from "../../core/util/Debug";
 
 export interface FFTOptions extends MeterBaseOptions {
 	size: PowerOfTwo;
@@ -77,4 +78,15 @@ export class FFT extends MeterBase<FFTOptions> {
 	set smoothing(val) {
 		this._analyser.smoothing = val;
 	}
+
+	/**
+	 * Returns the frequency value in hertz of each of the indices of the FFT's [[getValue]] response.
+	 * @example
+	 * const fft = new Tone.FFT(32);
+	 * console.log(fft.getIndexFrequency());
+	 */
+	getFrequencyOfIndex(index: number): Hertz {
+		assert(0 <= index && index < this.size, `index must be greater than or equal to 0 and less than ${this.size}`);
+		return index * this.context.sampleRate / (this.size * 2);
+	}
 }