Azure Functions JavaScript developer guide

The JavaScript experience for Azure Functions makes it easy to export a function, which is passed as a context object for communicating with the runtime and for receiving and sending data via bindings.

This article assumes that you've already read the Azure Functions developer reference.

Exporting a function

All JavaScript functions must export a single function via module.exports for the runtime to find the function and run it. This function must always include a context object.

// You must include a context, but other arguments are optional
module.exports = function(context) {
    // Additional inputs can be accessed by the arguments property
    if(arguments.length === 4) {
        context.log('This function has 4 inputs');
// or you can include additional inputs in your arguments
module.exports = function(context, myTrigger, myInput, myOtherInput) {
    // function logic goes here :)

Bindings of direction === "in" are passed along as function arguments, which means that you can use arguments to dynamically handle new inputs (for example, by using arguments.length to iterate over all your inputs). This functionality is convenient when you have only a trigger and no additional inputs, because you can predictably access your trigger data without referencing your context object.

The arguments are always passed along to the function in the order in which they occur in function.json, even if you don't specify them in your exports statement. For example, if you have function(context, a, b) and change it to function(context, a), you can still get the value of b in function code by referring to arguments[2].

All bindings, regardless of direction, are also passed along on the context object (see the following script).

context object

The runtime uses a context object to pass data to and from your function and to let you communicate with the runtime.

The context object is always the first parameter to a function and must be included because it has methods such as context.done and context.log, which are required to use the runtime correctly. You can name the object whatever you would like (for example, ctx or c).

// You must include a context, but other arguments are optional
module.exports = function(context) {
    // function logic goes here :)

context.bindings property


Returns a named object that contains all your input and output data. For example, the following binding definition in your function.json lets you access the contents of the queue from the context.bindings.myInput object.

// myInput contains the input data, which may have properties such as "name"
var author =;
// Similarly, you can set your output data
context.bindings.myOutput = { 
        some_text: 'hello world', 
        a_number: 1 };

context.done method


Informs the runtime that your code has finished. If your function uses the async function declaration (available using Node 8+ in Functions version 2.x), you do not need to use context.done(). The context.done callback is implicitly called.

If your function is not an async function, you must call context.done to inform the runtime that your function is complete. The execution will time out if it is missing.

The context.done method allows you to pass back both a user-defined error to the runtime and a property bag of properties that overwrite the properties on the context.bindings object.

// Even though we set myOutput to have:
//  -> text: hello world, number: 123
context.bindings.myOutput = { text: 'hello world', number: 123 };
// If we pass an object to the done function...
context.done(null, { myOutput: { text: 'hello there, world', noNumber: true }});
// the done method will overwrite the myOutput binding to be: 
//  -> text: hello there, world, noNumber: true

context.log method


Allows you to write to the streaming console logs at the default trace level. On context.log, additional logging methods are available that let you write to the console log at other trace levels:

Method Description
error(message) Writes to error level logging, or lower.
warn(message) Writes to warning level logging, or lower.
info(message) Writes to info level logging, or lower.
verbose(message) Writes to verbose level logging.

The following example writes to the console at the warning trace level:

context.log.warn("Something has happened."); 

You can set the trace-level threshold for logging in the host.json file, or turn it off. For more information about how to write to the logs, see the next section.

Binding data type

To define the data type for an input binding, use the dataType property in the binding definition. For example, to read the content of an HTTP request in binary format, use the type binary:

    "type": "httpTrigger",
    "name": "req",
    "direction": "in",
    "dataType": "binary"

Other options for dataType are stream and string.

Writing trace output to the console

In Functions, you use the context.log methods to write trace output to the console. At this point, you cannot use console.log to write to the console.

When you call context.log(), your message is written to the console at the default trace level, which is the info trace level. The following code writes to the console at the info trace level:

context.log({hello: 'world'});  

The preceding code is equivalent to the following code:{hello: 'world'});  

The following code writes to the console at the error level:

context.log.error("An error has occurred.");  

Because error is the highest trace level, this trace is written to the output at all trace levels as long as logging is enabled.

All context.log methods support the same parameter format that's supported by the Node.js util.format method. Consider the following code, which writes to the console by using the default trace level:

context.log('Node.js HTTP trigger function processed a request. RequestUri=' + req.originalUrl);
context.log('Request Headers = ' + JSON.stringify(req.headers));

You can also write the same code in the following format:

context.log('Node.js HTTP trigger function processed a request. RequestUri=%s', req.originalUrl);
context.log('Request Headers = ', JSON.stringify(req.headers));

Configure the trace level for console logging

Functions lets you define the threshold trace level for writing to the console, which makes it easy to control the way traces are written to the console from your functions. To set the threshold for all traces written to the console, use the tracing.consoleLevel property in the host.json file. This setting applies to all functions in your function app. The following example sets the trace threshold to enable verbose logging:

    "tracing": {      
        "consoleLevel": "verbose"     

Values of consoleLevel correspond to the names of the context.log methods. To disable all trace logging to the console, set consoleLevel to off. For more information, see host.json reference.

HTTP triggers and bindings

HTTP and webhook triggers and HTTP output bindings use request and response objects to represent the HTTP messaging.

Request object

The request object has the following properties:

Property Description
body An object that contains the body of the request.
headers An object that contains the request headers.
method The HTTP method of the request.
originalUrl The URL of the request.
params An object that contains the routing parameters of the request.
query An object that contains the query parameters.
rawBody The body of the message as a string.

Response object

The response object has the following properties:

Property Description
body An object that contains the body of the response.
headers An object that contains the response headers.
isRaw Indicates that formatting is skipped for the response.
status The HTTP status code of the response.

Accessing the request and response

When you work with HTTP triggers, you can access the HTTP request and response objects in any of three ways:

  • From the named input and output bindings. In this way, the HTTP trigger and bindings work the same as any other binding. The following example sets the response object by using a named response binding:

    context.bindings.response = { status: 201, body: "Insert succeeded." };
  • From req and res properties on the context object. In this way, you can use the conventional pattern to access HTTP data from the context object, instead of having to use the full pattern. The following example shows how to access the req and res objects on the context:

    // You can access your http request off the context ...
    if(context.req.body.emoji === ':pizza:') context.log('Yay!');
    // and also set your http response
    context.res = { status: 202, body: 'You successfully ordered more coffee!' }; 
  • By calling context.done(). A special kind of HTTP binding returns the response that is passed to the context.done() method. The following HTTP output binding defines a $return output parameter:

      "type": "http",
      "direction": "out",
      "name": "$return"

    This output binding expects you to supply the response when you call done(), as follows:

     // Define a valid response object.
    res = { status: 201, body: "Insert succeeded." };
    context.done(null, res);   

Node version and package management

The following table shows the Node.js version used by each major version of the Functions runtime:

Functions version Node.js version
1.x 6.11.2 (locked by the runtime)
2.x Active LTS and Current Node.js versions (8.11.1 and 10.6.0 recommended). Set the version by using the WEBSITE_NODE_DEFAULT_VERSION app setting.

You can see the current version that the runtime is using by printing process.version from any function.

The following steps let you include packages in your function app:

  1. Go to https://<function_app_name>

  2. Click Debug Console > CMD.

  3. Go to D:\home\site\wwwroot, and then drag your package.json file to the wwwroot folder at the top half of the page.
    You can upload files to your function app in other ways also. For more information, see How to update function app files.

  4. After the package.json file is uploaded, run the npm install command in the Kudu remote execution console.
    This action downloads the packages indicated in the package.json file and restarts the function app.

After the packages you need are installed, you import them to your function by calling require('packagename'), as in the following example:

// Import the underscore.js library
var _ = require('underscore');
var version = process.version; // version === 'v6.5.0'

module.exports = function(context) {
    // Using our imported underscore.js library
    var matched_names = _
        .where(context.bindings.myInput.names, {first: 'Carla'});

You should define a package.json file at the root of your function app. Defining the file lets all functions in the app share the same cached packages, which gives the best performance. If a version conflict arises, you can resolve it by adding a package.json file in the folder of a specific function.

Environment variables

To get an environment variable or an app setting value, use process.env, as shown here in the GetEnvironmentVariable function:

module.exports = function (context, myTimer) {
    var timeStamp = new Date().toISOString();

    context.log('Node.js timer trigger function ran!', timeStamp);   


function GetEnvironmentVariable(name)
    return name + ": " + process.env[name];

Considerations for JavaScript functions

When you work with JavaScript functions, be aware of the considerations in the following two sections.

Choose single-vCPU App Service plans

When you create a function app that uses the App Service plan, we recommend that you select a single-vCPU plan rather than a plan with multiple vCPUs. Today, Functions runs JavaScript functions more efficiently on single-vCPU VMs, and using larger VMs does not produce the expected performance improvements. When necessary, you can manually scale out by adding more single-vCPU VM instances, or you can enable auto-scale. For more information, see Scale instance count manually or automatically.

TypeScript and CoffeeScript support

Because direct support does not yet exist for auto-compiling TypeScript or CoffeeScript via the runtime, such support needs to be handled outside the runtime, at deployment time.

Next steps

For more information, see the following resources: