code: function defineConstants(constants) {
    const setter = context => prevConfig => {
      context.defineConstants = Object.assign({}, context.defineConstants, constants)
      return prevConfig
    }
  
    return Object.assign(setter, { post: addDefinePlugin })
  }
docs: Replaces constants in your source code with a value (`process.env.NODE_ENV`
  for example) using the `webpack.DefinePlugin`. Every constant's value is
  `JSON.stringify()`-ed first, so you don't have to remember.
  
  Using `defineConstants` multiple times results in a single
  DefinePlugin instance configured to do all the replacements.
  
  @param {object} constants  { [constantName: string]: * }
  @return {Function}