JavaScript: Add item to BEGINNING of object (NOT ARRAY)

How can I add an item to the beginning of an object?  This adds it to the end:

var obj = {"O":"-","R":"-","L":"-","D":"-"};
obj.W="-";
alert(JSON.stringify(obj));

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I was advised to use unshift() however that seems to only work with arrays.  This must stay an object, and the desired order must be maintained.