How/Where does JavaScript detect the default local

Question

Today, the 11th of September, 2017, JavaScript's toLocaleDateString() method is outputting 9/11/2017 for me. But I am in the UK, so the formatting is wrong in this case. The MDN Web Docs tell me that this method returns "a formatted string in the default locale".

So, where/how is JavaScript detecting my default locale? Where is this set, or what does it depend on, and (how) can I change it?

Edited to add: I'm using Firefox 55.0.3 on Windows 10 if that makes any difference.

Answer 1

To summarize shortly, detecting the current locale is implementation dependent and may differ from environment to environment. Your default language may also depend on the installer you've used to install your browser.

The not so short version:

Following the ECMAScript spec, conforming browsers (and other environments, such as Node.js) should implement localization following the ECMAScript Internationalization API (ECMA-402), which only outlines the following for getting the default locale:

The DefaultLocale abstract operation returns a String value representing the [...] language tag for the host environment’s current locale.

This means that getting the default locale is implementation dependent and can differ from browser to browser. This is intentional, as it allows browser vendors to keep their current, differing implementations to stay conforming without much fuss.

While it's true that it would be nice to have this standardized as well, it's more beneficial to get everyone on board for a broad spec first and then work out the little kinks later.

Most modern browsers allow you to change your current default locale in their preferences (Chrome shown):

Answer 2

You can pass locale as parameter. See the example below:

  var date = new Date().toLocaleDateString("en-GB");