2036318 - ObservableArray has null prototype but still behaves like an Array

Status: RESOLVED FIXED

(Core :: DOM: Bindings (WebIDL), defect)

Description

[Rob Wu [:robwu]]

An ObservableArray has a null prototype in Firefox (Array.prototype in Chrome and Safari):

console.log(document.adoptedStyleSheets.__proto__); // null
console.log(Object.getPrototypeOf(document.adoptedStyleSheets)); // null

Yet is still inherits from Array.prototype:

Array.prototype.customPush = function (s) { return this.push(s); };

sheet = new CSSStyleSheet();
sheet.replaceSync("*{background:pink}");
document.adoptedStyleSheets.customPush(sheet); // page turns pink

Even after replacing its prototype, methods are still exposed from Array.prototype instead of whatever the prototype was set to (but somehow DevTools' autocompletion suggests method from the specified prototype):

Array.prototype.customPush2 = function (s) { return this.push(s); };
Object.setPrototypeOf(document.adoptedStyleSheets, { test: 1 });
console.log(document.adoptedStyleSheets.__proto__); // { test: 1 }
console.log(document.adoptedStyleSheets.test); // undefined (1 in Chrome+Safari)
console.log(document.adoptedStyleSheets.customPush2); // function (undefined in Chrome+Safari)

sheet = new CSSStyleSheet();
sheet.replaceSync("*{background:red}");
document.adoptedStyleSheets.customPush2(sheet); // page turns red (error in Chrome+Safari)

I found this while looking at bug 1751346; this is a follow-up to https://phabricator.services.mozilla.com/D291049#10323557

Comment 1

[Rob Wu [:robwu]]

Attachment: Bug 2036318 - Specify prototype for observable arrays

Comment 2

[Pulsebot]

Pushed by [e-mail]:
https://github.com/mozilla-firefox/firefox/commit/b0f3671e754b
https://hg.mozilla.org/integration/autoland/rev/934066f82a0f
Specify prototype for observable arrays r=jandem

Comment 3

[Cosmin Sabou [:CosminS]]

https://hg.mozilla.org/mozilla-central/rev/934066f82a0f