Bug 138859 – global function declaration should be throwing a TypeError if trying to override a readonly property

• Component: JavaScriptCore
• Status: NEW

[Daejun Park]

According to ES5, Section 10.5 Declaration Binding Instantiation, Step 5.e.iv,
http://es5.github.io/#x10.5

a duplicate global function declaration throws a TypeError exception,
when the existing property is not configurable and:
- an accessor descriptor,
- or, not writable,
- or, not enumerable.

For example, the following code is supposed to throw a TypeError exception for each duplicate function declaration, "f0", "f1", "f2", and "f3":

Object.defineProperty(this, "f0", { "get" : undefined, "set" : undefined, "enumerable" : false, "configurable" : false });
eval(" function f0() { return 0; } "); // TypeError

Object.defineProperty(this, "f1", { "value" : 0,      "writable" : false, "enumerable" : false, "configurable" : false });
eval(" function f1() { return 0; } "); // TypeError

Object.defineProperty(this, "f2", { "value" : 0,      "writable" : true,  "enumerable" : false, "configurable" : false });
eval(" function f2() { return 0; } "); // TypeError

Object.defineProperty(this, "f3", { "value" : 0,      "writable" : false, "enumerable" : true,  "configurable" : false });
eval(" function f3() { return 0; } "); // TypeError


However, Safari does not report any exception, while Chrome and Firefox correctly throw the TypeError exceptions.

I've tested this using the Web Inspector console of Safari 7.0.4.

Thanks,
Daejun

[Oliver Hunt]

Retitling to reflect actual bug - it's not due to a duplicate function. We're short circuiting creation of a new global property when we decide to shadow and that means we skip the readonly check on the existing property. Whoops.

[Radar WebKit Bug Importer]

<rdar://problem/19032865>

[Satish Srinivasan]

I think this is already fixed in 355e9d25900a300375b1b68576f926278d8b5903. JavaScriptCore from Safari Version 26.1 (21622.2.11.11.9) does throw TypeError for all four cases.