在使用该类中的方法时引用类中的对象

Question

我正在研究一种简单的键盘记录器，它使用javascript记录用户击键，我可以将其用于各种应用程序。我有一个主类，它有一个对象，它基本上是一个子对象列表，对应键盘上的各种键。每个条目包含键的ID和表示是否按下它的布尔值。我在类中也有一个init函数，在所有加载后立即调用。并且init类基本上通过输入对象运行，并将其与任何键盘事件进行比较，以查看是否有任何ID匹配。我能够在init函数的顶部引用该对象，但是当我尝试在事件函数中引用输入对象时，它返回undefined。我完全不知道为什么会这样，我会非常感谢一些帮助。

function Controls() {
this.input = {
    A: {ID: 65, active: false},
    B: {ID: 66, active: false},
    C: {ID: 67, active: false},
    D: {ID: 68, active: false},
    E: {ID: 69, active: false},
    F: {ID: 70, active: false},
    G: {ID: 71, active: false},
    H: {ID: 72, active: false},
    I: {ID: 73, active: false},
    J: {ID: 74, active: false},
    K: {ID: 75, active: false},
    L: {ID: 76, active: false},
    M: {ID: 77, active: false},
    N: {ID: 78, active: false},
    O: {ID: 79, active: false},
    P: {ID: 80, active: false},
    Q: {ID: 81, active: false},
    R: {ID: 82, active: false},
    S: {ID: 83, active: false},
    T: {ID: 84, active: false},
    U: {ID: 85, active: false},
    V: {ID: 86, active: false},
    W: {ID: 87, active: false},
    X: {ID: 88, active: false},
    Y: {ID: 89, active: false},
    Z: {ID: 90, active: false},
    ZERO: {ID: 48, active: false},
    ONE: {ID: 49, active: false},
    TWO: {ID: 50, active: false},
    THREE: {ID: 51, active: false},
    FOUR: {ID: 52, active: false},
    FIVE: {ID: 53, active: false},
    SIX: {ID: 54, active: false},
    SEVEN: {ID: 55, active: false},
    EIGHT: {ID: 56, active: false},
    NINE: {ID: 57, active: false},
    GRAVE: {ID: 192, active: false},
    DASH: {ID: 189, active: false},
    EQUALS: {ID: 187, active: false},
    OPENBRAKET: {ID: 219, active: false},
    CLOSEBRACKET: {ID: 221, active: false},
    BACKSLASH: {ID: 220, active: false},
    SEMICOLON: {ID: 186, active: false},
    QUOTE: {ID: 222, active: false},
    COMMA: {ID: 188, active: false},
    PERIOD: {ID: 190, active: false},
    FORWARDSLASH: {ID: 191, active: false},
    F1: {ID: 112, active: false},
    F2: {ID: 113, active: false},
    F3: {ID: 114, active: false},
    F4: {ID: 115, active: false},
    F5: {ID: 116, active: false},
    F6: {ID: 117, active: false},
    F7: {ID: 118, active: false},
    F8: {ID: 119, active: false},
    F9: {ID: 120, active: false},
    F10: {ID: 121, active: false},
    F11: {ID: 122, active: false},
    F12: {ID: 123, active: false},
    ESCAPE: {ID: 27, active: false},
    INSERT: {ID: 45, active: false},
    DELETE: {ID: 46, active: false},
    BACKSPACE: {ID: 8, active: false},
    TAB: {ID: 9, active: false},
    CAPSLOCK: {ID: 20, active: false},
    SHIFT: {ID: 16, active: false},
    CONTROL: {ID: 17, active: false},
    WINDOWSKEY: {ID: 91, active: false},
    ALT: {ID: 18, active: false},
    ENTER: {ID: 13, active: false},
    WINDOWSMENU: {ID: 93, active: false},
    UPARROW: {ID: 38, active: false},
    LEFTARROW: {ID: 37, active: false},
    DOWNARROW: {ID: 40, active: false},
    RIGHTARROW: {ID: 39, active: false},
    SPACEBAR: {ID: 32, active: false}
}

this.init = function() {
    document.onkeydown = function() {
        var event = window.event;
        console.log(this.input);
        for (i in this.input) {
            if (this.input[i].ID.toString() == event.keyCode) {
                this.input[i].active = true;
            }
        }
    }

    document.onkeyup = function() {
        var event = window.event;
        for (i in this.input) {
            if (this.input[i].ID.toString() == event.keyCode) {
                this.input[i].active = false;
            }
        }
    }
}

}