preload, play, and crossfade audio tracks in p5js

sketch.js

var audio = [];
var numTracks = 4;

function preload() {
	audio[0] = loadSound('/audio/001.mp3');
	audio[1] = loadSound('/audio/002.mp3');
	audio[2] = loadSound('/audio/003.mp3');
	audio[3] = loadSound('/audio/004.mp3');
}

function setup() {
	createCanvas( windowWidth, windowHeight );
	noLoop();
}

function draw() {}

function fadeTrack(tid, time) {
	console.log("fading: " + tid);
	audio[tid].setVolume(0, time);
	setTimeout(function() {
		audio[tid].stop();
	}, (time*1000)+250);
}

function playTrack(tid, volume, time) {
	console.log("playing: " + tid);
	if( !audio[tid].isPlaying() ) {
		audio[tid].play();
		audio[tid].setVolume(0);
		setTimeout(function(){
			audio[tid].setVolume(volume, time);
		}, 100);
	}
}

function keyPressed() {
	var key = keyCode - 49;
	
	if( key >= 0 && key < numTracks ) {
		console.log( "key pressed: " + key );
		for( var i = 0; i < numTracks; i++ ) {
			if( i != key ) {
				if( audio[i].isPlaying() ) {
					fadeTrack( i, 2 );		
				}
				
			}
		}

		if( !audio[key].isPlaying() ) {
			playTrack( key, .3, 5 );	
		}
	}
}