mellowsoon mellowsoon - 3 months ago 11x
Javascript Question

Does JavaScript Guarantee Object Property Order?

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?


No, properties order in objects is not guaranteed in JavaScript; you need to use an Array.

Definition of an Object from ECMAScript Third Edition (pdf):

4.3.3 Object

An object is a member of the type Object. It is an unordered collection of properties each of which contains a primitive value, object, or function. A function stored in a property of an object is called a method.

Since ECMAScript 2015, using the Map object could be an alternative. A Map shares some similarities with an Object and guarantees the keys order:

A Map iterates its elements in insertion order, whereas iteration order is not specified for Objects.