Does JavaScript Guarantee Object Property Order?

Question

If I create an object like this:

var obj = {};
obj.prop1 = "Foo";
obj.prop2 = "Bar";

Will the resulting object always look like this?

{ prop1 : "Foo", prop2 : "Bar" }

That is, will the properties be in the same order that I added them?

Answer 1

From the JSON standard:

An object is an unordered collection of zero or more name/value pairs, where a name is a string and a value is a string, number, boolean, null, object, or array.

(emphasis mine).

So, no you can't guarantee the order.

Answer 2

In modern browsers you can use the Map data structure instead of a object.

Developer mozilla > Map

A Map object can iterate its elements in insertion order...