//for store any predefined value or static value in widget configuration editor
<preference name="key" value="value" readonly="false"/> 

//for read/write data in code
var key = "key",
    value;
if (typeof widget.preferences[key] !== "undefined") {
    //read data
    value = widget.preferences.getItem(key); 
}

//write data
widget.preferences.setItem(key, value);