The environment variables are embedded during the build time. Since Create React App produces a static HTML/CSS/JS bundle, it can’t possibly read them at runtime. To read them at runtime, you would need to load HTML into memory on the server and replace placeholders in runtime, asdescribed here. Alternatively you can rebuild the app on the server anytime you change them.

Note: You must create custom environment variables beginning withREACT_APP_. Any other variables exceptNODE_ENV will be ignored to avoid accidentallyexposing a private key on the machine that could have the same name. Changing any environment variables will require you to restart the development server if it is running.

These environment variables will be defined for you onprocess.env. For example, having an environment variable namedREACT_APP_NOT_SECRET_CODE will be exposed in your JS asprocess.env.REACT_APP_NOT_SECRET_CODE.

There is also a built-in environment variable calledNODE_ENV. You can read it fromprocess.env.NODE_ENV. When you runnpm start, it is always equal to'development', when you runnpm test it is always equal to'test', and when you runnpm run build to make a production bundle, it is always equal to'production'.You cannot overrideNODE_ENV manually. This prevents developers from accidentally deploying a slow development build to production.

These environment variables can be useful for displaying information conditionally based on where the project is deployed or consuming sensitive data that lives outside of version control.

First, you need to have environment variables defined. For example, let’s say you wanted to consume an environment variable inside a<form>:

render(){
return(
<div>
<small>You are runningthis applicationin<b>{process.env.NODE_ENV}</b> mode.</small>
<form>
<inputtype="hidden"defaultValue={process.env.REACT_APP_NOT_SECRET_CODE}/>
</form>
</div>
);
}

During the build,process.env.REACT_APP_NOT_SECRET_CODE will be replaced with the current value of theREACT_APP_NOT_SECRET_CODE environment variable. Remember that theNODE_ENV variable will be set for you automatically.

When you load the app in the browser and inspect the<input>, you will see its value set toabcdef, and the bold text will show the environment provided when usingnpm start:

<div>
<small>You are running this application in<b>development</b> mode.</small>
<form>
<inputtype="hidden"value="abcdef"/>
</form>
</div>

The above form is looking for a variable calledREACT_APP_NOT_SECRET_CODE from the environment. In order to consume this value, we need to have it defined in the environment. This can be done using two ways: either in your shell or in a.env file. Both of these ways are described in the next few sections.

Having access to theNODE_ENV is also useful for performing actions conditionally:

if(process.env.NODE_ENV!=='production'){
  analytics.disable();
}

When you compile the app withnpm run build, the minification step will strip out this condition, and the resulting bundle will be smaller.

Referencing Environment Variables in the HTML​

Note: this feature is available withreact-scripts@0.9.0 and higher.

You can also access the environment variables starting withREACT_APP_ in thepublic/index.html. For example:

<title>%REACT_APP_WEBSITE_NAME%</title>

Note that the caveats from the above section apply:

• Apart from a few built-in variables (NODE_ENV andPUBLIC_URL), variable names must start withREACT_APP_ to work.
• The environment variables are injected at build time. If you need to inject them at runtime,follow this approach instead.

Adding Temporary Environment Variables In Your Shell​

Defining environment variables can vary between OSes. It’s also important to know that this manner is temporary for the life of the shell session.

Windows (cmd.exe)​

set "REACT_APP_NOT_SECRET_CODE=abcdef" && npm start

(Note: Quotes around the variable assignment are required to avoid a trailing whitespace.)

Windows (Powershell)​

($env:REACT_APP_NOT_SECRET_CODE = "abcdef") -and (npm start)

Linux, macOS (Bash)​

REACT_APP_NOT_SECRET_CODE=abcdef npm start

Adding Development Environment Variables In.env​

Note: this feature is available withreact-scripts@0.5.0 and higher.

To define permanent environment variables, create a file called.env in the root of your project:

REACT_APP_NOT_SECRET_CODE=abcdef

Note: You must create custom environment variables beginning withREACT_APP_. Any other variables exceptNODE_ENV will be ignored to avoidaccidentally exposing a private key on the machine that could have the same name. Changing any environment variables will require you to restart the development server if it is running.

Note: You need to restart the development server after changing.env files.

.env filesshould be checked into source control (with the exclusion of.env*.local).

What other.env files can be used?​

Note: this feature isavailable withreact-scripts@1.0.0 and higher.

• .env: Default.
• .env.local: Local overrides.This file is loaded for all environments except test.
• .env.development,.env.test,.env.production: Environment-specific settings.
• .env.development.local,.env.test.local,.env.production.local: Local overrides of environment-specific settings.

Files on the left have more priority than files on the right:

• npm start:.env.development.local,.env.local,.env.development,.env
• npm run build:.env.production.local,.env.local,.env.production,.env
• npm test:.env.test.local,.env.test,.env (note.env.local is missing)

These variables will act as the defaults if the machine does not explicitly set them.

Please refer to thedotenv documentation for more details.

Note: If you are defining environment variables for development, your CI and/or hosting platform will most likely needthese defined as well. Consult their documentation how to do this. For example, see the documentation forTravis CI orHeroku.

Expanding Environment Variables In.env​

Note: this feature is available withreact-scripts@1.1.0 and higher.

Expand variables already on your machine for use in your.env file (usingdotenv-expand).

For example, to get the environment variablenpm_package_version:

REACT_APP_VERSION=$npm_package_version
# also works:
# REACT_APP_VERSION=${npm_package_version}

Or expand variables local to the current.env file:

DOMAIN=www.example.com
REACT_APP_FOO=$DOMAIN/foo
REACT_APP_BAR=$DOMAIN/bar