JavaScript Developer Guide for Azure Functions

Introduction to JavaScript in Azure Functions

Azure Functions provides a powerful serverless compute experience for Node.js developers. This guide covers key concepts, best practices, and reference information for building and deploying JavaScript functions.

Key Features

• Event-driven execution: Functions are triggered by events, such as HTTP requests, timer schedules, or messages from service queues.
• Scalability: Azure Functions automatically scales to meet demand, ensuring your applications remain available and performant.
• Language Support: Full support for Node.js with access to the npm ecosystem.
• Bindings: Declarative way to connect to other Azure services and data sources without writing integration code.

Writing Your First JavaScript Function

A typical JavaScript function in Azure Functions has the following structure:

module.exports = async function (context, req) {
    context.log('JavaScript HTTP trigger function processed a request.');

    const name = (req.query.name || (req.body && req.body.name));
    const responseMessage = name
        ? 'Hello, ' + name + '!'
        : 'This HTTP triggered function executed successfully. Pass a name in the query string or in the request body for a personalized response.';

    context.res = {
        status: 200, /* Defaults to 200 */
        body: responseMessage
    };
};

Understanding the Function Signature

• context: An object that provides information about the function's execution, including logging capabilities and output bindings.
• req: The incoming request object. For HTTP triggers, this contains request details like headers, query parameters, and the request body.
• module.exports = async function (...): This standard Node.js export makes your function callable by the Azure Functions runtime.

Context Object

The context object is central to your function's execution:

Properties:

• context.log(...): For logging information during execution.
• context.bindingData: Access to binding parameters.
• context.res: Used to define the HTTP response for HTTP-triggered functions.

Methods:

• context.done(): (Optional) Call to signal completion. Async functions implicitly handle completion.

Request Object (HTTP Trigger)

The req object for an HTTP trigger provides:

• req.method: The HTTP method (e.g., 'GET', 'POST').
• req.query: An object containing query string parameters.
• req.body: The parsed request body.
• req.headers: An object containing request headers.
• req.params: Route parameters defined in the function's route.

Response Object (HTTP Trigger)

Set context.res to define the HTTP response:

context.res = {
    status: 200,
    headers: {
        'Content-Type': 'application/json'
    },
    body: {
        message: 'Success!'
    }
};

Common HTTP Response Properties:

• status: The HTTP status code.
• body: The response payload.
• headers: An object of response headers.

Working with Bindings

Bindings simplify integration with other services. They are configured in function.json. Here's an example of a function triggered by HTTP and writing to a Queue Storage output binding:

// function.json
{
  "bindings": [
    {
      "authLevel": "function",
      "type": "httpTrigger",
      "direction": "in",
      "name": "req",
      "methods": [
        "get",
        "post"
      ]
    },
    {
      "type": "http",
      "direction": "out",
      "name": "res"
    },
    {
      "type": "queue",
      "direction": "out",
      "name": "outputQueueItem",
      "queueName": "myqueue-items",
      "connection": "AzureWebJobsStorage"
    }
  ]
}
            

module.exports = async function (context, req) {
    context.log('HTTP trigger function processed a request.');

    const message = req.body.message || 'Default message';

    // The output binding receives the value
    context.bindings.outputQueueItem = message;

    context.res = {
        status: 200,
        body: `Message "${message}" sent to queue.`
    };
};
            

Environment Variables and Configuration

Configuration settings, including connection strings, are managed through application settings. These are available as environment variables within your function execution context.

const connectionString = process.env.MyServiceConnectionString;
context.log(`Connection string: ${connectionString}`);
            

Local Development and Testing

Use the Azure Functions Core Tools for local development and testing. This allows you to run and debug your functions on your local machine before deploying to Azure.

• Install Core Tools: npm install -g azure-functions-core-tools@4 --unsafe-perm true
• Initialize a project: func init MyProject --worker-runtime node
• Create a function: func new --template "HTTP template" --name MyHttpFunction
• Run the project: func start

Next Steps

• Explore different binding types.
• Learn about trigger options.
• Understand deployment strategies.