Using Custom CLI Parameters in Node

index.md

Using Custom CLI Parameters in Node

In the following TypeScript example, we're expecting two possible CLI parameters, --build and --port. We prepare a paramMap variable that has default values for all parameters, which are overwritten for each parameter that is manually passed in via the CLI.

See further down for a TypeScript example without comments as well as a JavaScript example.

// create an enum that contains each of the params
enum Params {
  BUILD = '--build',
  PORT = '--port',
}

// create a paramMap that's a Record type where the keys are Param enums, and their
// values are strings. paramMap is equal to the return value of Object.assign().
const paramMap: Record<Params, string> = Object.assign({}, {
    // We start with an empty object {} for Object.assign().
    // Then, we merge in an object containing default values for the params: 'dist' & '1000'.
    [Params.BUILD]: 'dist',
    [Params.PORT]: '1000',
  },
  // We then create an array from Node's process.argv array by slicing the first two items from the array
  // and getting the remaining items. Since process.argv always starts with 1) process.execPath (the
  // absolute pathname of the executable that started the Node process), and 2) the path of the
  // JavaScript file being executed, we can slice those arguments off, leaving us with just custom
  // arguments that the user passes in.

  // We then call reduce() on this array, taking each argument and splitting it into a key/value param.
  process.argv.slice(2).reduce((obj, param) => {
    const [key, value] = param.split('=');
    // If the key is in the Params enum, then we set this key & value on a new object.
    // Once the reduce is finished, it's passed in as the final object to Object.assign(), overwriting
    // default params.
    if (Object.values(Params).includes(key as Params)) obj[key] = value;
    return obj;
  }, {}),
);

// pull off the params to be used as local variables `build` & `port`
const {
  [Params.BUILD]: build,
  [Params.PORT]: port,
} = paramMap;

Without comments:

enum Params {
  BUILD = '--build',
  PORT = '--port',
}

const paramMap: Record<Params, string> = Object.assign({}, {
    [Params.BUILD]: 'dist',
    [Params.PORT]: '1000',
  },
  process.argv.slice(2).reduce((obj, param) => {
    const [key, value] = param.split('=');
    if (Object.values(Params).includes(key as Params)) obj[key] = value;
    return obj;
  }, {}),
);

const {
  [Params.BUILD]: build,
  [Params.PORT]: port,
} = paramMap;

Just JavaScript:

const Params = {
  BUILD: '--build',
  PORT: '--port',
};

const paramMap = Object.assign({}, {
    [Params.BUILD]: 'dist',
    [Params.PORT]: '1000',
  },
  process.argv.slice(2).reduce((obj, param) => {
    const [key, value] = param.split('=');
    if (Object.values(Params).includes(key)) obj[key] = value;
    return obj;
  }, {}),
);

const {
  [Params.BUILD]: build,
  [Params.PORT]: port,
} = paramMap;