Finding the cursor keys in Animate javascript

I've searched  the forums for an answer but can't find what I'm looking for. I'm trying to convert some AS3 code to javaScript and I can't figure out the syntax for the left and right cursor keys. This is what I've got in AS3:

function keyDownFunction(event : KeyboardEvent) : void
{

if (event.keyCode == 39)
{
walkerSpeed = walkerSpeed+1;
}
if (event.keyCode == 37)
{
walkerSpeed = walkerSpeed-1;

}
}