Freezing object properties or array values, whilst keeping them extensible

Allen Wirfs-Brock allen at
Tue Jun 4 09:29:39 PDT 2013

Object.defineProperty(myArray, "length", {writable: false});

A writable, non-configurable property can still be set to non-writable.


On Jun 4, 2013, at 6:57 AM, Andy Earnshaw wrote:

> Something that occurred to me today is that we have methods for locking down objects in different ways except for preventing changes to existing properties.  We have:
>  - Object.seal to prevent new properties being added or existing properties being deleted
>  - Object.preventExtensions to prevent new properties being added
>  - Object.freeze to prevent new properties being added or existing properties being altered/deleted
> Let's say, though, that my library has a function that accepts an object as an argument, performs some changes to the values of properties on that object and returns it to the developer's code for use in a different function provided by my library.  If I didn't want the developer changing any of the properties the function sets, I'd have to copy all properties across to a new object.  That's all well and good, but what if it's an array?  The length property of an array isn't configurable, so you can't prevent it being written without freezing it which, in turn, prevents extensions altogether.
> Array.create might have taken care of that, depending on how it was implemented, but it seems as though that's been deferred in favour of the <| operator proposal.
> If it's not too late for ES6 proposals, I'd like to suggest that we add either an function to freeze only existing properties of an object, e.g. Object.lock(), or a function to freeze arrays, e.g. Array.freeze().  The latter would only freeze the length property and existing array index properties, keeping it extensible.
> Other than introducing another aspect of control over the library user, I could see this being useful as an optimisation; if the array elements are locked and you can confirm it was locked by your code then you could make assumptions about the last element in the array based on an inspection of the first, eliminating the need for a loop.
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