# Introduction

## What are Layers?​

Layers (a.k.a. Universes) allow us to create experiments that are mutually exclusive to each other. Each layer has a logical representation of all your users and can have experiments created "within" this layer. Users that are in one experiment of a layer, cannot also be in another experiment in the same layer.

You can add experiments to a layer (or create a layer) during experiment creation.

Once you create a layer, you'll be able to manage them on the layer management tab under Experiments.

In addition to that, Layers are key to improving engineering efficiency and iteration velocity for product teams. In a Layer, parameters exist at the Layer level, and can be shared across experiments within the Layer. Due to this characteristic, we can abstract the concept of "Experiment" away from the SDKs so that users only need to deal with parameters in code, which makes it super easy to run multiple experiments that change the same thing and iterate on the same experiment without any code changes.

Let's say your product has an important sign up dialog, which contains some text that your team runs a lot of tests on, some of which were ran in parallel, and some were iterations of previous experiments. If you work with Experiments directly, your code will look like this over time:

let signUpText = DEFAULT_SIGNUP_TEXT;const signUpTestV1 = statsig.getExperiment('sign_up_dialog_text_test_v1');const signUpTestV2 = statsig.getExperiment('sign_up_dialog_text_test_v2');const specialSignUpTest = statsig.getExperiment('sign_up_test_special_offer');const holidaySignUpTest = statsig.getExperiment('sign_up_test_holiday');if (signUpTestV1.get('is_in_test', false)) {    // original test, added in app version v1.2    signUpText = signUpTestV1.get('dialog_content', DEFAULT_SIGNUP_TEXT);} else if (signUpTestV2.get('is_in_test', false)) {    // v2 of the original test, added in app version v1.6 because we wanted to test a new copy but don't want to stop v1    signUpText = signUpTestV2.get('dialog_content', DEFAULT_SIGNUP_TEXT);} else if (specialSignUpTest.get('is_in_test', false)) {    // test showing a special offer in the text, added in v2.0    signUpText = specialSignUpTest.get('dialog_content', DEFAULT_SIGNUP_TEXT);} else if (holidaySignUpTest.get('is_in_test', false)) {    // test showing some holiday greetings in the dialog, added in v2.1    signUpText = holidaySignUpTest.get('dialog_content', DEFAULT_SIGNUP_TEXT);}// Then we display the text in the dialog

Every time you add a new test, you need to change the code and it's only available in a new version.

However, things become A LOT easier if you work with Layers:

let signUpText = statsig.getLayer('sign_up_tests').get('sign_up_dialog_text', DEFAULT_SIGNUP_TEXT);// Then we display the text in the dialog

That's all the code you ever need! No more code changes and app releases for new tests. Every time you want to add a new test, simply add a new experiment to the same Layer and choose the parameter sign_up_dialog_text as a parameter for the new experiment. The SDK takes care of figuring out which value to serve for the user, based on which experiment the user is allocated to.

## A Word on Exposures​

When calling getLayer(LayerName<string>) you won't see any exposure logged, however, you will see an exposure logged when accessing a specific parameter within the layer using getLayer(LayerName<string>).get(Parameter<string>). The name of the exposure event is called statsig::layer_exposure.

• If the user is assigned to an experiment within the Layer, the statsig::layer_exposure exposure event is billable.
• If the user is not assigned an experiment within the Layer, the statsig::layer_exposure exposure event is not billable.

You will see multiple exposure events logged for each parameters accessed, but these are de-duped on Statsig servers.

## See it in Action​

To learn about all that you can do with Layers with an E2E example, see the Statsig tutorial on Layers in JS.