Source code

Revision control

Copy as Markdown

Other Tools

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at */
#ifndef js_Prefs_h
#define js_Prefs_h
#include "js/PrefsGenerated.h"
// [SMDOC] Prefs
// JS::Prefs is used to make JS preferences defined in StaticPrefList.yaml
// available to SpiderMonkey code.
// Adding a Pref
// =============
// Adding a new pref is easy. For example, if you're adding a new JS feature,
// you could add the following to StaticPrefList.yaml:
// - name: javascript.options.experimental.my_new_feature
// type: bool
// value: false
// mirror: always
// set_spidermonkey_pref: startup
// The value of this pref can then be accessed in SpiderMonkey code with
// |JS::Prefs::experimental_my_new_feature()|.
// The default pref value in the YAML file applies to all SpiderMonkey builds
// (browser, JS shell, jsapi-tests, etc), so by default this feature will be
// disabled everywhere.
// To enable your feature, use the |--setpref experimental.my_new_feature=true|
// JS shell command line argument, or set the browser pref in about:config.
// Because this is a 'startup' pref, a browser restart is required for this to
// take effect.
// The rest of this comment describes more advanced use cases.
// Non-startup prefs
// =================
// Setting |set_spidermonkey_pref = startup| is recommended for most prefs.
// In this case the pref is only set during startup so we don't have to worry
// about the pref value changing at runtime.
// However, for some prefs this doesn't work. For instance, the WPT test harness
// can set test-specific prefs after startup. To properly update the JS pref in
// this case, |set_spidermonkey_pref = always| must be used. This means the
// SpiderMonkey pref will be updated whenever it's changed in the browser.
// Setting Prefs
// =============
// Embedders can override pref values. For startup prefs, this should only be
// done during startup (before calling JS_Init*) to avoid races with worker
// threads and to avoid confusing code with unexpected pref changes:
// JS::Prefs::setAtStartup_experimental_my_new_feature(true);
// Non-startup prefs can also be changed after startup:
// JS::Prefs::set_experimental_my_new_feature(true);
// JS Shell Prefs
// ==============
// The JS shell |--list-prefs| command line flag will print a list of all of the
// available JS prefs and their current values.
// To change a pref, use |--setpref name=value|, for example
// |--setpref experimental.my_new_feature=true|.
// It's also possible to add a custom shell flag. In this case you have to
// override the pref value yourself based on this flag.
// Testing Functions
// =================
// The |getAllPrefNames()| function will return an array with all JS pref names.
// The |getPrefValue(name)| function can be used to look up the value of the
// given pref. For example, use |getPrefValue("experimental.my_new_feature")|
// for the pref defined above.
namespace JS {
class Prefs {
// For each pref, define a static |pref_| member.
#ifdef DEBUG
static void assertCanSetStartupPref();
static void assertCanSetStartupPref() {}
// For each pref, define static getter/setter accessors.
static TYPE CPP_NAME() { return CPP_NAME##_; } \
static void SETTER(TYPE value) { \
assertCanSetStartupPref(); \
} \
CPP_NAME##_ = value; \
* Specification for whether weak refs should be enabled and if so whether the
* FinalizationRegistry.cleanupSome method should be present.
enum class WeakRefSpecifier {
inline WeakRefSpecifier GetWeakRefsEnabled() {
if (!Prefs::weakrefs()) {
return WeakRefSpecifier::Disabled;
if (Prefs::experimental_weakrefs_expose_cleanupSome()) {
return WeakRefSpecifier::EnabledWithCleanupSome;
return WeakRefSpecifier::EnabledWithoutCleanupSome;
}; // namespace JS
#endif /* js_Prefs_h */