如何有条件地修改 ES6 Javascript 模块的导出

ESM 不支持条件导出模式。

为了使用来自另一个模块的动态导入来修改导出的值，该模块的说明符派生自环境变量（在尝试...catch 语句，以便失败的尝试不会在顶层抛出未捕获的异常），您可以修改导出的结构，以便将它们公开为对象上的属性。下面是一个可重现的例子来演示：

./package.json:

{
  "name": "so-77465699",
  "version": "0.1.0",
  "type": "module",
  "scripts": {
    "dev": "NODE_ENV=Dev node main.js",
    "prod": "NODE_ENV=production node main.js"
  },
  "license": "MIT"
}

./Dev.Config.js:

export const Config = {
  DatabaseHosts: ["localhost"],
  DatabasePort: 5500,
};

export default Config;

./Config.js:

const Config = {
  DatabaseName: "myDbName",
  DatabasePort: 3000,
  DatabaseHosts: ["174.292.292.32"],
  MaxWebRequest: 50,
  MaxImageRequests: 50,
  WebRequestTimeout: 30,
};

try {
  // Import a module specifier based on
  // the value of the NODE_ENV environemnt variable.
  // Destructure and rename the default export:
  const { default: envConfig } = await import(
    import.meta.resolve(`./${process.env.NODE_ENV}.Config.js`)
  );

  // Iterate the keys and values, updating the existing Config object:
  for (const [key, value] of Object.entries(envConfig)) {
    Config[key] = value;
  }
} catch (cause) {
  console.log(`Error overriding config with local values: ${cause}`);
}

export { Config, Config as default };

./main.js:

// Import named export:
import { Config } from "./Config.js";

// Alternatively, since it's also the default export:
// import { default as Config } from "./Config.js";

// Or, using syntax sugar:
// import Config from "./Config.js";

console.log(Config.DatabaseHosts);

在终端中：

% node --version
v20.9.0

% npm run dev

> [email protected] dev
> NODE_ENV=Dev node main.js

[ 'localhost' ]

% npm run prod

> [email protected] prod
> NODE_ENV=production node main.js

Error overriding config with local values: Error [ERR_MODULE_NOT_FOUND]: Cannot find module '/Users/node/so-77465699/production.Config.js' imported from /Users/node/so-77465699/Config.js
[ '174.292.292.32' ]

在 ES6 模块中无法动态构建/覆盖导出对象。即使是有条件的导入也已经需要顶级或工具支持。await

但是，您可以做的是弄乱声明和：leteval

export const DatabaseName = "myDbName"; // keep things as `const` to prevent overriding them
export let DatabasePort = 3000;
export let DatabaseHosts = ["174.292.292.32"];
export let MaxWebRequest = 50;
export let MaxImageRequests = 50;
export const WebRequestTimeout = 30;
… // etc

try {
    var environmentConfig = `./${process.env.NODE_ENV}.Config.js`;
    var localConfig = await import(environmentConfig)
    for (const [name, value] of Object.entries(localConfig)) {
        eval(`${name} = value;`);
    }
} catch (error) {
    console.log("Error overriding config with local values. " + error)
}

不过我不推荐这个。仅当无法更改 Config.js 模块的使用方式，或者导出的变量太多而替代方法不可行时，才使用此方法。

相反，我建议您创建一个单独的模块来合并配置，尽管这需要拼写两次配置名称：

// Default.Config.js
export const DatabaseName = "myDbName";
export const DatabasePort = 3000;
export const DatabaseHosts = ["174.292.292.32"];
export const MaxWebRequest = 50;
export const MaxImageRequests = 50;
export const WebRequestTimeout = 30;

// Dev.Config.js
export const DatabaseHosts = ["localhost"];
export const DatabasePort = 5500;

// Config.js
import * as defaultConfig from "./Default.Config.js";

const localConfig = await import(`./${process.env.NODE_ENV}.Config.js`).catch(error => {
    console.log("Error overriding config with local values. " + error);
    return {};
});

export const {
    DatabaseName,
    DatabasePort,
    DatabaseHosts,
    MaxWebRequest,
    MaxImageRequests,
    WebRequestTimeout,
    … // etc
} = Object.assign({}, defaultConfig, localConfig);

或者将配置模块更改为默认导出对象，您可以任意操作这些对象。但是，您失去了使用命名导入和获得静态验证的能力。