Most applications require certain configuration values that can be changed at runtime of the app. The environment.ts files in an Angular application technically work for setting configuration values in an app, but those are buildtime configuration values. This means that they are set when the application is built, and can't be changed unless the app is built again.

This library provides an easy way to load one or more JSON files with configuration values or make one or more HTTP GET calls to an API endpoint that returns those values. The config objects that are returned from the call(s) will be combined into a single configuration object. You can then use that configuration throughout the application. The default location of the JSON file is in the assets folder, at ./assets/config.json. When the service loads the file, it stores that configuration object in a local variable which can be accessed via the getConfig() and getConfigObjectKey(key: string) methods. getConfig returns the entire configuration object; getConfigObjectKey(key: string) returns part of the configuration object, namely the part defined by the key passed in. In some cases, the config.json is not finished loading before other modules/services are, so the above methods will return null. If that is the case, subscribe to the configSubject and access the configuration object that way.

In your app.module.ts file, add the following to the @NgModule decorator:

imports: [..., RuntimeConfigLoaderModule, ...],

That's it; it's that simple. In the RuntimeConfigLoaderModule, the APP_INITIALIZER token is used to run a function which loads the configuration from a file or an API endpoint that can be used throughout the application.

If you implement the library exactly as it is above, the configuration file needs to be in the ./assets/config.json location as mentioned above. If you'd like to load the file from a different location, provide that location in the .forRoot() method when importing the RuntimeConfigLoaderModule:

imports: [
  ...,
  RuntimeConfigLoaderModule.forRoot(
    { configUrl: './path/to/config/config.json' }
  ),
  ...]

If you want to load multiple files, the value of configUrl should be an array of strings:

imports: [
  ...,
  RuntimeConfigLoaderModule.forRoot(
    { configUrl: ['./path/to/config/config-1.json', './path/to/config/config-2.json'] }
  ),
  ...]

Make sure that the path(s) you provide here is accessible by the Angular application, meaning that the file is somewhere the app can load it. In my opinion, the assets folder is the easiest place to work from.

One reason you may want to load multiple configuration objects is so that you can set the configuration on your machine without affecting anyone else. For example, you could have a local.config.json file that is not included in source control. Some of the values in that file would overwrite the values in a config file that everyone can use. Another use case is that some config values don't change between environments, and some do. The ones that don't change could go in one file, the ones that do change could go in a second file. Each developer/environment can provide the second file with values they want or need.

It's important to know that if an attribute is repeated in two configuration files, the latest value is kept. So, let's say you have apiUrl in both files, config-1.json and config-2.json. Let's assume the files are passed in to the forRoot method like this:

imports: [
  ...,
  RuntimeConfigLoaderModule.forRoot(
    { configUrl: ['./path/to/config/config-1.json', './path/to/config/config-2.json'] }
  ),
  ...]

In this case, the apiUrl value from config-2.json will override the value from config-1.json.