在SomeObj对象中，onkeydown事件处理程序this.doSomething在错误的上下文中被调用（textbox元素的上下文中），但它需要在this的上下文中被调用。如何做到这一点？

function SomeObj(elem1, elem2) {
    this.textboxElem = elem1;
    this.someElem = elem2;
    this.registerEvent();
}

SomeObj.prototype = {
    registerEvent: function() {
        this.textboxElem.onkeydown = this.doSomething;
    },
    doSomething: function() {
        // this must not be textboxElem
        alert(this);
        this.someElem.innerHTML = "123";
    }
};