点击键盘左键，没反应，我的keydown怎么没调用啊

$(document).keydown(function (event){

switch( event.keyCode ){

case 37://left

if(moveLeft()){

generateOneNumber();

isgameover();

}

break;

case 38://top

if(moveTop()){

generateOneNumber();

isgameover();  

}

break;

case 39://right

if(moveRight()){

generateOneNumber();

isgameover();

}

break;

case 40://bottom

if(moveBottom()){

generateOneNumber();

isgameover();

}

break;

default:

break;

}

});