The chrome.preferences module contains a prototype for giving other modules access to manage preferences stored by Chrome. This prototype is used, for example, for chrome.proxy.settings.

Overview

The preferences API provides a common set of functions (get(), set(), and clear()) as well as an event publisher (onChange) for various preferences in Chrome. The proxy settings examples demonstrate how these functions are intended to be used.

Scope and life cycle of preferences

Chrome distinguishes between three different scopes of preferences:

regular

Preferences set in the regular scope apply to regular browser windows and are inherited by the incognito windows if they are not overwritten. These preferences are stored to disk and remain in place until they are cleared by the governing extension, or the governing extension is disabled or uninstalled.

incognito_persistent

Preferences set in the incognito_persistent scope apply only to incognito windows. For these, they override regular preferences. These preferences are stored to disk and remain in place until they are cleared by the governing extension, or the governing extension is disabled or uninstalled.

incognito_session_only

Preferences set in the incognito_session_only scope apply only to incognito windows. For these, they override regular and incognito_session_only preferences. These preferences are not stored to disk and are cleared when the last incognito window is closed. They can only be set when at least one incognito window is open.

Precedence

Chrome manages preferences on different layers. The following list describes the layers that may influence the effective preferences, in increasing order of precedence.

1. System preferences provided by the operating system
2. Command-line parameters
3. Preferences set by extensions
4. Policies

As the list implies, policies might overrule any changes that you specify with your extension. You can use the get() function to determine whether your extension is capable of setting a preference or whether this setting would be overridden.

As discussed above, Chrome allows using different preferences for regular windows and incognito windows. The following example illustrates the behavior. Assume that no policy overrides the preferences and that an extension can set preferences for regular windows (R) and preferences for incognito windows (I).

• If only (R) is set, these preferences are effective for both regular and incognito windows.
• If only (I) is set, these preferences are effective for only incognito windows. Regular windows use the preferences determined by the lower layers (command-line options and system settings).
• If both (R) and (I) are set, the respective preferences are used for regular and incognito windows.

If two or more extensions want to set the same preference to different values, the extension installed most recently takes precedence over the other extensions. If the most recently installed extension sets only (I), the preference of regular windows can be defined by previously installed extensions.

The effective preference value is the setting that results from considering the preference rules. It is used by Chrome.