monosynth.js

    function noteFromKey(keyCode) {
        var char = String.fromCharCode(keyCode).toUpperCase();
        if(pianoMap[char]) {
            return pianoMap[char];
        }
        else {
            return false;
        }
    }
/*
    "c4" : 260.74, 
    "c#4": 278.44,
    "d4" : 293.33, 
    "d#4": 309.03,
    "e4" : 330.00, 
    "f4" : 347.65, 
    "f#4": 371.25,
    "g4" : 391.11, 
    "g#4": 417.66,
    "a4" : 440.000,  
    "a#4": 463.54,
    "b4" : 495.00, 
    "c5" : 521.48,
*/
    var pianoMap = {
        "Z":  260.74, // C-4
        "S": 278.44, // #
        "X":  293.33, // D-4
        "D": 309.03, // #
        "C":  330.00, // E-4
        "V":  347.65, // F-4
        "G": 371.25, // #
        "B":  391.11, // G-4
        "H": 417.66, // #
        "N":  440.00, // A-4 
        "J": 463.54, // #
        "M":  495.00, // B-4
        "Q":  521.48, // C-5

    }; 
    
    // declare our objects ----
    var ctx   = new AudioContext();
    var osc   = ctx.createOscillator();
    var amp   = ctx.createGain();
    
    // configure properties ----
    amp.gain.value = 0;
    
    // connect objects ----
    osc.connect(amp);
    amp.connect(ctx.destination);
    
    // start oscillator ----
    osc.start();

    // ----------------
    // key events ----
    window.onkeydown = function(e) {
        var key = noteFromKey(e.keyCode);
        if(!key) {return;}
        console.log(osc,key);
        osc.frequency.setValueAtTime(key, osc.context.currentTime);
        amp.gain.value = .01;
    };
    
    window.onkeyup = function() {
        amp.gain.value = 0;
    };