Testing

Javascript clousure is one of the common reason to make a function much less testable. Submodule comes prepared. We introduced hijacking in version 4.1 of the library. As such, it is easy to change the actual implementation to a mock implementation to test

For example

import { create } from "@submodule/core"
type LogLevel = 'debug' | 'log' | 'error'
const config = create(() => ({ logLevel: 'debug' as LogLvel } /** imagine this config is loaded from .env */))
 
const logger = create((config) => {
  return (...msgs: string[]) => {
    if (config.logLevel === 'debug') {
      ...
    } else if ...
  }
}, config)

As you see from the code, the implementation of the logger heavy dependent on the config value. To test, you can easily extract the implementation of logger then write unit test for it, sometime, it is just a hassle to do sometime

Instead, you can also control the transfered config using hijacking

import { value } from "@submodule/core"
logger._inject(value({ logLevel: 'error' })

With this, the dependent value will be as fed and you can simulate other cases