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  • Some Array methods don't consider array holes and length

Some Array methods don't consider array holes and length

# Yusuke Suzuki (13 years ago)

Hello,

In some Array methods such as 'slice', 'splice', they don't [[Put]] 'length' value. And they check [[HasProperty]] before [[DefineOwnProperty]]. As the result, they waste empty trailing elements, but it is not compatible behavior to ES3.

For example,

[0,,,,].slice(0)

In ES3, this result is [0,,,,] and length is 4. But, in ES5.1 algorithm step 10-k, kPresent is true when only the first case comes, so result A is [0] and length is 1.

All the modern engines behave like ES3, so I think this is issue of ES5.1, is it right?

The issue of the same kind was found before in Array.prototype.concat, and I reported it. mail.mozilla.org/pipermail/es5-discuss/2010-December/003851.html

And A of Array.prototype.splice has the same problem. In section 15.4.4.12 step 9-c, we check fromPresent and [[DefineOwnProperty]] to A, but we don't perform [[Put]] length operation to A.

For example,

[0,,,,].splice(0, 4)

In ES3, this result is [0,,,] and length is 4. But, in ES5.1 algorithm step 9-c, fromPresent is true when only the first case comes, so result A is [0] and length is 1.

So I suggest updating spec template that returns Array.

P.S. Sorry for duplicate mails, they're reflected lately, so I thought they are filtered.

, Yusuke Suzuki

# Allen Wirfs-Brock (13 years ago)

On Jun 25, 2012, at 1:18 AM, Yusuke Suzuki wrote:

Hello,

In some Array methods such as 'slice', 'splice', they don't [[Put]] 'length' value. And they check [[HasProperty]] before [[DefineOwnProperty]]. As the result, they waste empty trailing elements, but it is not compatible behavior to ES3.

For splice, this has already been reported as ecmascript#332 concat is bug ecmascript#129

I've added a new bug for slice: ecmascript#417

Are there any other cases like these that you have identified?

# Yusuke Suzuki (13 years ago)

For splice, this has already been reported as ecmascript#332

Oh, I missed it, thanks!! I've added myself to CC.

I've added a new bug for slice:

ecmascript#417

Thanks for filing this.

Are there any other cases like these that you have identified?

As long as I know, all cases have been already reported. For example, old Array.from spec had the same problem. But in the latest draft, [[Put]] 'length' is inserted, so this issue doesn't occur. gist.github.com/1074126

, Yusuke Suzuki

# Rick Waldron (13 years ago)

On Tue, Jun 26, 2012 at 9:01 AM, Yusuke Suzuki <utatane.tea at gmail.com>wrote:

For splice, this has already been reported as

ecmascript#332

Oh, I missed it, thanks!! I've added myself to CC.

I've added a new bug for slice:

ecmascript#417

Thanks for filing this.

Are there any other cases like these that you have identified?

As long as I know, all cases have been already reported. For example, old Array.from spec had the same problem. But in the latest draft, [[Put]] 'length' is inserted, so this issue doesn't occur. gist.github.com/1074126

Sorry for any confusion - I wrote that as a "proof of concept". The draft that Allen produces should be considered the canonical document to follow for spec developments moving towards ES6 standardization.