Serverless Frameworkにおける環境変数の外部ファイル化の一例

/conf/dev/dev.json

{
  "Other_File": "Development environment"
}

serverless.ts

import type { AWS } from '@serverless/typescript';

// 関数間で共通の環境変数はこちらに定義
const commonEnvironmentVariables = {
  Other_File: "${self:custom.otherfile.${self:provider.stage}.Other_File}",
};

const serverlessConfiguration: AWS = {
  service: "create-category-collections-api",
  frameworkVersion: "2",
  custom: {
    webpack: {
      webpackConfig: "./webpack.config.js",
      includeModules: true,
    },
    otherfile: {
      dev: "${file(./conf/dev/dev.json)}",
      stg: "${file(./conf/stg/stg.json)}",
      prd: "${file(./conf/prd/prd.json)}",
    },
  },
  // Add the serverless-webpack plugin
  plugins: ["serverless-webpack", "serverless-jest-plugin"],
  provider: {
    name: "aws",
    runtime: "nodejs12.x",
    apiGateway: {
      minimumCompressionSize: 1024,
    },
    environment: {
      AWS_NODEJS_CONNECTION_REUSE_ENABLED: "1",
      ...commonEnvironmentVariables,
    },
    region: "ap-northeast-1",
    stage: "${opt:stage, self:custom.defaultStage}",
  },
  functions: {
    hello: {
      handler: "handler.hello",
      events: [
        {
          http: {
            method: "get",
            path: "hello",
          },
        },
      ],
    },
  },
};

module.exports = serverlessConfiguration;

参考

Serverless Frameworkで環境変数を外部ファイルから読み込み、環境毎に自動で切り替えてみる
https://dev.classmethod.jp/articles/serverless-framework-conf-change/