# Environments URL: https://docs.ablyft.com/developers/installation/environments/ > Run one experiment on several websites, languages or stages, and decide with a rule where it applies. Environments let you run the same experiment on several websites, or on different versions of one website, without duplicating your project. Your pages, audiences and goals are defined once and reused everywhere. Typical examples: * A live site and a staging site * Regional or language sites, for example `us`, `uk` and `ca` * One site that is served on several subdomains ## How it works 1. **Create** an environment in your project and give it a rule: a short piece of JavaScript that returns `true` on the sites where the environment applies. 2. **Assign** the environment to your experiments. 3. **At runtime**, the snippet first checks the [page](https://docs.ablyft.com/guides/audiences-and-delivery/pages/) conditions. Then, for each experiment, it checks the rules of the experiment's environments and afterwards the [audiences](https://docs.ablyft.com/guides/audiences-and-delivery/audiences/). See the [snippet sequence](https://docs.ablyft.com/developers/installation/snippet-sequence/). An experiment without environment is not restricted. With several environments, one matching rule is enough. The full matching table is in the [environments guide](https://docs.ablyft.com/guides/audiences-and-delivery/environments/#assign-environments-to-an-experiment). An empty rule always matches. ## Create and assign environments Creating an environment (fields, API name, default environment, code helper) and assigning it to experiments is described in the [environments guide](https://docs.ablyft.com/guides/audiences-and-delivery/environments/#create-an-environment). The **API Name** is used in the file name of the [sub-snippet](#sub-snippets). ## Write the rule The rule is the body of a JavaScript function. Return `true` if the environment applies, otherwise `false` (or nothing). ```js // The environment applies on every page return true; ``` Inside the rule you can use the [tools](https://docs.ablyft.com/developers/reference/javascript-api/tools/), available as `ablyftTools`. [`ablyftTools.parseUrl()`](https://docs.ablyft.com/developers/reference/javascript-api/tools/#parseurl) splits the current address into `subdomain`, `domain` and `tld` (and more). For `www.example.com` the result is `www`, `example` and `com`. **By subdomain.** For a site like `en.mystore.com`: ```js return ablyftTools.parseUrl().subdomain === 'en'; ``` **By domain and top-level domain.** For regional shops on different domains: ```js const { domain, tld } = ablyftTools.parseUrl(); return domain === 'mystore' && tld === 'co.uk'; ``` **Live versus staging.** ```js // Live: the production host. Staging: everything else. return ablyftTools.parseUrl().hostname === 'www.mystore.com'; ``` **By an attribute in the page**, such as the language of the HTML document: ```js return document.documentElement.lang === 'de'; ``` Things to keep in mind: * The rule is checked repeatedly for about **2 seconds**, so it can also depend on content that appears shortly after the page loads. * If the rule throws an error, the error is logged and the environment counts as **not matching**. * Keep rules short and without side effects. They run on every page view. If the same experiment should run on several domains, make sure its pages do not depend on one domain, see [Match types](https://docs.ablyft.com/guides/audiences-and-delivery/pages/#match-types). ## Sub-snippets By default, the [main snippet](https://docs.ablyft.com/developers/installation/snippet/) contains all running experiments and every experiment checks its own environment rules. If you enable **Create a sub-snippet** for an environment, ABlyft additionally creates a snippet that contains **only the experiments of that environment**. Use a sub-snippet if you want to load less data on a specific site. You find it under **Settings → Snippet → Sub-Snippets**. The name contains the API name of the environment: ```html ``` The environment rules are still checked in a sub-snippet. Include either the main snippet or a sub-snippet on a page, not both: a second ABlyft snippet on the same page is ignored. ## Edit and delete Editing and deleting environments, and the roles required, are described in the [environments guide](https://docs.ablyft.com/guides/audiences-and-delivery/environments/#edit-or-delete-an-environment). ## Troubleshooting If an experiment does not run on a site and its page matches, check the environment next, because it is tested before the audiences: 1. Does the experiment have environments assigned? If so, does the rule of at least one return `true` on this page? 2. Does the rule throw an error? Errors count as "not matching". 3. Do the pages of the experiment use relative URLs? 4. Turn on debug mode and open the browser console. It shows whether an environment is valid. Only if at least one environment is valid, the snippet continues with the audiences. See [Debugging](https://docs.ablyft.com/developers/debugging/). ## API access Environments of a project can be read through the REST API (`projects/{project}/environments`, read only). Experiment endpoints accept an `environments` list with environment IDs. See the [REST API](https://docs.ablyft.com/developers/reference/rest-api/).