Bug 163338 - Array.prototype.slice should not modify frozen objects
Summary: Array.prototype.slice should not modify frozen objects
Alias: None
Product: WebKit
Classification: Unclassified
Component: JavaScriptCore (show other bugs)
Version: WebKit Local Build
Hardware: Unspecified Unspecified
: P2 Normal
Assignee: Mark Lam
Depends on:
Reported: 2016-10-12 10:01 PDT by Mark Lam
Modified: 2016-10-12 11:31 PDT (History)
6 users (show)

See Also:

proposed patch. (7.65 KB, patch)
2016-10-12 10:11 PDT, Mark Lam
fpizlo: review+
Details | Formatted Diff | Diff

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Description Mark Lam 2016-10-12 10:01:46 PDT
The ES6 spec for Array.prototype.slice (https://tc39.github.io/ecma262/#sec-array.prototype.slice) states that it uses the CreateDataPropertyOrThrow() (https://tc39.github.io/ecma262/#sec-createdatapropertyorthrow) to add items to the result array.  The spec for CreateDataPropertyOrThrow states:

"This abstract operation creates a property whose attributes are set to the same defaults used for properties created by the ECMAScript language assignment operator. Normally, the property will not already exist. If it does exist and is not configurable or if O is not extensible, [[DefineOwnProperty]] will return false causing this operation to throw a TypeError exception."

Array.prototype.slice also uses a Set function (https://tc39.github.io/ecma262/#sec-set-o-p-v-throw) to set the "length" property and passes true for the Throw argument.  Ultimately, it ends up calling the OrdinarySet function (https://tc39.github.io/ecma262/#sec-ordinaryset) that will fail if the property is not writable.  This failure should result in a TypeError being thrown in Set.

Since the properties of frozen objects are not extensible, not configurable, and not writeable, Array.prototype.slice should fail to write to the result array if it is frozen.
Comment 1 Mark Lam 2016-10-12 10:11:55 PDT
Created attachment 291364 [details]
proposed patch.
Comment 2 Mark Lam 2016-10-12 11:31:09 PDT
Thanks for the review.  Landed in r207226: <http://trac.webkit.org/r207226>.