How do I loop through or enumerate a JavaScript object?

I have a JavaScript object like the following:

var p = {
    "p1": "value1",
    "p2": "value2",
    "p3": "value3"
};

Now I want to loop through all p elements (p1, p2, p3…) And get their keys and values. How can I do that?

I can modify the JavaScript object if necessary. My ultimate goal is to loop through some key value pairs and if possible I want to avoid using eval.