serverless/lib/plugins/aws/dev/local-lambda/runtime-wrappers/node/javascript-wrapper.js

import path from 'path';
import os from 'os';
import fs from 'fs';
import { inspect } from 'util';

/**
 * Custom log function that formats objects with colors regardless of their depth.
 *
 * @param {*} originalFunction Original console function to be modified
 * @returns
 */
function customLog(originalFunction) {
  return function (...args) {
    args = args.map((arg) => {
      if (typeof arg === 'object' && arg !== null) {
        // Format the object with colors and unlimited depth
        return inspect(arg, { colors: true, depth: null });
      }
      return arg;
    });
    originalFunction.apply(console, args);
  };
}

/**
 * We apply the custom log function only to the console.log and console.info functions.
 * console.error and console.warn are printed in red and handled by the parent process.
 * We don't need to handle process.stdout.write and process.stderr.write
 * Because they can't don't accept objects anyway.
 */
console.log = customLog(console.log);
console.info = customLog(console.info);

/**
 * Saves the invocation result to a temporary file in JSON format.
 * The file is named with the process ID to avoid conflicts between concurrent invocations.
 * The result includes both the response and the error if any.
 *
 * @param {*} result
 * @returns {void}
 */
const saveInvocationResult = (result) => {
  const filePath = path.join(os.tmpdir(), `sls_${process.pid}.json`);

  const stringifiedResult = JSON.stringify(result, null, 2);

  fs.writeFileSync(filePath, stringifiedResult);
};

/**
 * Imports the user handler function from the specified file path.
 * We first try to require the file, and if it fails with ERR_REQUIRE_ESM, we import it.
 * This supports both CommonJS and ESM handler files.
 * If the file is already cached, it is deleted to ensure the latest version is imported
 * as the user makes changes during the dev mode session
 *
 * @param {string} handlerFileAbsolutePath - Path to handler file
 * @param {string} handlerName - The Handler Name defined in the config file
 * @returns {Promise<Function>}
 */
const importFunction = async (handlerFileAbsolutePath, handlerName) => {
  let handlerModule;
  try {
    handlerModule = await import(handlerFileAbsolutePath);
  } catch (error) {
    throw error;
  }

  // Make sure the handler is a function
  const handlerFunction = handlerModule[handlerName];
  if (typeof handlerFunction !== 'function') {
    throw new Error(`Handler is not a function`);
  }

  return handlerFunction;
};

/**
 * Invoke the handler function with the event, context and callback.
 * We add context functions to the context object we received via WebSockets.
 *
 * @param {*} handlerFunction The handler function to invoke
 * @param {{ event: any, partialContext: any }} options
 * @returns <Promise<any>} a promise that resolves with the response from the handler function
 * @throws {Error} if the handler function is not a function, or if the handler function throws an error
 */
const invokeFunction = async (handlerFunction, { event, partialContext }) => {
  return new Promise(async (resolve, reject) => {
    const callback = (error, response) => {
      if (error) {
        reject(error);
      } else {
        resolve(response);
      }
    };

    const startTime = new Date();

    const context = {
      ...partialContext,
      succeed(result) {
        callback(null, result);
      },
      fail(error) {
        callback(error, null);
      },
      done(error, result) {
        callback(error, result);
      },
      getRemainingTimeInMillis() {
        return Math.max(context.timeout * 1000 - (new Date().valueOf() - startTime.valueOf()), 0);
      },
    };

    try {
      const response = await handlerFunction(event, context, callback);
      resolve(response);
    } catch (error) {
      reject(error);
    }
  });
};

/**
 * We parse all the data passed in by the parent process
 */
const inputArgs = JSON.parse(process.argv[2]);
const { handlerFileAbsolutePath, handlerName, event, partialContext } = inputArgs;

(async () => {
  try {
    // Import and invoke the handler function
    const handler = await importFunction(handlerFileAbsolutePath, handlerName);
    const response = await invokeFunction(handler, { event, partialContext });

    // Save the response for the parent process to fetch and return to Lambda
    saveInvocationResult({ response, error: null });
  } catch (error) {
    // Save the error for the parent process to fetch and return to Lambda
    saveInvocationResult({
      response: null,
      error: { name: error.name, message: error.message, stack: error.stack },
    });

    // Exit the child process in case of errors
    console.error(error);
    process.exit(1);
  }
})();