Set up an Incoming Webhook in Slack

• Go to your Slack workspace and create an Incoming Webhook URL. This will allow you to post messages to a Slack channel.

• Save the Webhook URL for use in your Lambda function.

Create an AWS Lambda Function

• Go to the AWS Management Console and create a new Lambda function.

• AWS Lambda Function Runtime settings:

  • Runtime: Node.js 20.x (for fetch API availability - see https://aws.amazon.com/jp/blogs/compute/node-js-18-x-runtime-now-available-in-aws-lambda/)

  • Handler: index.handler

  • Architecture: arm64

  • Update runtime version: Auto

Write the Lambda Function Code

Use the following JavaScript code to create your Lambda function. Copy it to your Lambda function code's index.mjs file.

This function sends an alert message to the specified Slack channel using the Incoming Webhook URL and the fetch API.

// Example: 'https://hooks.slack.com/services/zzz/xxx'
const SLACK_WEBHOOK_URL = process.env.SLACK_WEBHOOK_URL;

export const handler = async (event) => {
  // Log the received event for debugging purposes
  console.log('Event received:', JSON.stringify(event));

  const message =
    event.message ?? 'Alert! Check your systems. (default message from Lambda)';

  const result = await sendAlertToSlack(message);

  return {
    statusCode: result.statusCode,
    body: result.message,
  };
};

const sendAlertToSlack = async (message) => {
  try {
    const response = await fetch(SLACK_WEBHOOK_URL, {
      method: 'POST',
      headers: {
        'Content-Type': 'application/json',
      },
      body: JSON.stringify({ text: message }),
    });

    if (!response.ok) {
      const errorText = await response.text();

      console.error(
        'Error sending message to Slack:',
        response.status,
        errorText,
      );

      return { statusCode: response.status, message: errorText };
    }

    console.log('Successfully sent message to Slack:', message);

    return { statusCode: 200, message: 'OK' };
  } catch (error) {
    console.error('Error with request:', error);
    return { statusCode: 500, message: error.message };
  }
};

May be you can see a warning on Lambda Function Code editor at the fetch API.

fetch is not defined. Please fix or add global fetch.

This warning you can ignore because you are using the new fetch API of new version Node.js.

Here is a tip.

• On development environment:

  • You can change mjs file extension to ts on local for better type checking.

  • Create a new package manager and install eslint to lint your code.

pnpm init
# or
bun init

• View all source code on browser AWS lambda function editor to check if there are warnings in your code.

Set Environment Variables

• In the AWS Lambda console, go to your function's configuration and add an environment variable with the name SLACK_WEBHOOK_URL and the value as your Slack Webhook URL.

• If no need to keep it secret, set SLACK_WEBHOOK_URL variable directly.

Deploy and Test the Function

• To save the source code, deploy the Lambda function.

• Test the function by creating a test event with a JSON payload, for example:

{
  "message": "This is a test alert from AWS Lambda."
}

• Press test button and you should see a message at Slack channel.

Set Up Trigger (Optional)

If you want this Lambda function to be triggered by specific events (e.g., CloudWatch alarms, SNS messages), configure the appropriate trigger in the AWS Lambda console.

Keep the source code by git repository with formatter and linter configuration

You can choose prettier, eslint or biome. For example:

biome.json

{
  "$schema": "https://biomejs.dev/schemas/1.8.3/schema.json",
  "organizeImports": {
    "enabled": false
  },
  "javascript": {
    "formatter": {
      "quoteStyle": "single"
    }
  },
  "formatter": {
    "indentStyle": "space"
  }
}