Shifting from Express.js to Azure Functions

Express.js is one of the most popular Node.js frameworks for web developers and remains an excellent choice for building apps that serve API endpoints.

When migrating code to a serverless architecture, refactoring Express.js endpoints affects the following areas:

  • Middleware: Express.js features a robust collection of middleware. Many middleware modules are no longer required in light of Azure Functions and Azure API Management capabilities. Ensure you can replicate or replace any logic handled by essential middleware before migrating endpoints.

  • Differing APIs: The API used to process both requests and responses differs among Azure Functions and Express.js. The following example details the required changes.

  • Default route: By default, Azure Functions endpoints are exposed under the api route. Routing rules are configurable via routePrefix in the host.json file.

  • Configuration and conventions: A Functions app uses the function.json file to define HTTP verbs, define security policies, and can configure the function's input and output. By default, the folder name that which contains the function files defines the endpoint name, but you can change the name via the route property in the function.json file.


Learn more through the interactive tutorial Refactor Node.js and Express APIs to Serverless APIs with Azure Functions.



The following example shows a typical Express.js GET endpoint.

// server.js
app.get('/hello', (req, res) => {
  try {
  } catch(error) {
    const err = JSON.stringify(error);
    res.status(500).send(`Request error. ${err}`);

When a GET request is sent to /hello, an HTTP 200 response containing Success is returned. If the endpoint encounters an error, the response is an HTTP 500 with the error details.

Azure Functions

Azure Functions organizes configuration and code files into a single folder for each function. By default, the name of the folder dictates the function name.

For instance, a function named hello has a folder with the following files.

| - hello
|  - function.json
|  - index.js

The following example implements the same result as the above Express.js endpoint, but with Azure Functions.

// hello/index.js
module.exports = async function (context, req) {
  try {
    context.res = { body: "Success!" };
  } catch(error) {
    const err = JSON.stringify(error);
    context.res = {
      status: 500,
      body: `Request error. ${err}`

When moving to Functions, the following changes are made:

  • Module: The function code is implemented as a JavaScript module.

  • Context and response object: The context allows you to communicate with the Function's runtime. From the context, you can read request data and set the function's response. Synchronous code requires you to call context.done() to complete execution, while asyc functions resolve the request implicitly.

  • Naming convention: The folder name used to contain the Azure Functions files is used as the endpoint name by default (this can be overridden in the function.json).

  • Configuration: You define the HTTP verbs in the function.json file such as POST or PUT.

The following function.json file holds configuration information for the function.

  "bindings": [
      "authLevel": "function",
      "type": "httpTrigger",
      "direction": "in",
      "name": "req",
      "methods": ["get"]
      "type": "http",
      "direction": "out",
      "name": "res"

By defining get in the methods array, the function is available to HTTP GET requests. If you want to your API to accept support POST requests, you can add post to the array as well.

Next steps