Configuration with app.json / app.config.js
app.json
is your go-to place for configuring parts of your app that don't belong in code. It is located at the root of your project next to your package.json
. It looks something like this:
{
"expo": {
"name": "My app",
"slug": "my-app"
}
}
Most configuration from
app.json
is accessible at runtime from your JavaScript code via
Constants.manifest
. Sensitive information such as secret keys are removed. See the
"extra"
key below for information about how to pass arbitrary configuration data to your app.
app.json
configures many things, from your app name to icon to splash screen and even deep linking scheme and API keys to use for some services. To see a full list of available properties, please refer to the
app.json / app.config.js reference.
💡
Do you use Visual Studio Code? If so, we recommend that you install the
vscode-expo extension to get auto-completion of properties in
app.json
files.
For more customization you can use the JavaScript and TypeScript app.config.js
, or app.config.ts
. These configs have the following properties:
- Comments, variables, and single quotes!
- ES module support (import/export).
- TypeScript support with nullish coalescing and optional chaining.
- Updated whenever Metro bundler reloads.
- Provide environment information to your app.
- Does not support Promises.
For example, you can export an object as default to define your config in app.config.js
:
const myValue = 'My App';
export default {
name: myValue,
version: process.env.MY_CUSTOM_PROJECT_VERSION || '1.0.0',
extra: {
fact: 'kittens are cool',
},
};
Extras can be accessed via expo-constants
:
import Constants from 'expo-constants';
Constants.manifest.extra.fact === 'kittens are cool';
You can access and modify incoming config values by exporting a function that returns an object. This is useful if your project also has an app.json
. By default, Expo CLI will read the app.json
first and send the normalized results to the app.config.js
. This functionality is disabled when the --config
is used to specify a custom config.
For example, your app.json
could look like this:
{
"expo": {
"name": "My App"
}
}
And in your app.config.js
, you are provided with that configuration in the arguments to the exported function:
export default ({ config }) => {
console.log(config.name);
return {
...config,
};
};
⚠️
This is experimental and subject to breaking changes.
You can use autocomplete and doc-blocks with a TypeScript config app.config.ts
. Install the unversioned typings for Expo config with yarn add -D @expo/config
and create an app.config.ts
with the following contents:
import { ExpoConfig, ConfigContext } from '@expo/config';
export default ({ config }: ConfigContext): ExpoConfig => ({
...config,
name: 'My App',
});
There are two different types of configs: static (app.config.json
, app.json
), and dynamic (app.config.js
, app.config.ts
). Static configs can be automatically updated with CLI tools, whereas dynamic configs must be manually updated by the developer.
- The static config is read if
app.config.json
exists (falls back to app.json
). If no static config exists, then default values are inferred from the package.json
and your dependencies. - The dynamic config is read if either
app.config.ts
or app.config.js
exist. If both exist, then the TypeScript config is used. - If the dynamic config returns a function, then the static config is passed to the function with
({ config }) => ({})
. This function can then mutate the static config values. - The return value from the dynamic config is used as the final config. It cannot have any promises.
- All functions in the config are evaluated and serialized before any tool in the Expo ecosystem uses it. The config must be a JSON manifest when it is hosted.