Miss any of our Open RFC calls?Watch the recordings here! »


2.3.0 • Public • Published


Talkback is a javascript HTTP proxy to record and playback HTTP requests. As long as you have node.js in your environment you can run talkback to record requests from applications written in any language/framework.
You can use it to accelerate your integration tests or run your application against a mocked server.

npm version Build Status


npm install talkback


Talkback is pretty easy to setup.
Define which host it will be proxying, which port it should listen to and where to find and save tapes.

When a request arrives to talkback, it will try to match it against a previously saved tape and quickly return the tape's response.
If no tape matches the request, it will forward it to the origin host, save the tape to disk for future uses and return the response.

const talkback = require("talkback");
//import talkback from "talkback/es6";
const opts = {
  host: "https://api.myapp.com/foo",
  record: talkback.Options.RecordMode.NEW,
  port: 5544,
  path: "./my-tapes"
const server = talkback(opts);
server.start(() => console.log("Talkback Started"));

Talkback can be used in 2 ways:

  • as a standalone HTTP server in its own separate process. Example.
  • as a library, where you are in charge of routing requests to talkback. Example.

talkback(options: Partial<Options>): TalkbackServer

Returns an unstarted instance of a talkback server.
See all Options.

const talkback = talkback(options)
talkback.start(() => console.log("Talkback Started"))

talkback.requestHandler(options: Partial<Options>): Promise<RequestHandler>

Returns a RequestHandler instance ready to receive requests.
See all Options.

The handler takes a request and returns a response Promise.

const talkbackHandler = await talkback.requestHandler(options)
const response = await talkbackHandler.handle(httpRequest)


Name Type Description Default
host String Where to proxy unknown requests
port String Talkback port 8080
path String Path where to load and save tapes ./tapes/
https Object HTTPS server options Defaults
record String \| Function Set record mode. More info RecordMode.NEW
fallbackMode String \| Function Fallback mode for unknown requests when recording is disabled. More info FallbackMode.NOT_FOUND
name String Server name Defaults to host value
latency String \| Array \| Function Synthetic latency for requests (in ms). More info 0
errorRate Number \| Function Probabilty between 0 and 100 of injecting a synthetic error. More info 0
tapeNameGenerator Function Customize how a tape name is generated for new tapes. null
ignoreHeaders [String] List of headers to ignore when matching tapes. Useful when having dynamic headers like cookies or correlation ids ['content-length', 'host]
ignoreQueryParams [String] List of query params to ignore when matching tapes. Useful when having dynamic query params like timestamps []
ignoreBody Boolean Should the request body be ignored when matching tapes false
bodyMatcher Function Customize how a request's body is matched against saved tapes. More info null
urlMatcher Function Customize how a request's URL is matched against saved tapes. More info null
requestDecorator Function Modify requests before they're proxied. More info null
responseDecorator Function Modify responses before they're returned. More info null
silent Boolean Disable requests information console messages in the middle of requests false
summary Boolean Enable exit summary of new and unused tapes at exit. More info true
debug Boolean Enable verbose debug information false

HTTPS options

Name Type Description Default
enabled Boolean Enables HTTPS server false
keyPath String Path to the key file null
certPath String Path to the cert file null


Tapes are where talkback stores requests and their response.
They can be freely edited to match new requests or return a different response than the original. They are loaded recursively from the path directory at startup.
Since they are only loaded on startup, any changes to a tape requires a server restart to be applied.
Tapes use the JSON5 format. JSON5 is an extensions to the JSON format that allows for very neat features like comments, trailing commas and keys without quotes.


All tapes have the following 3 properties:

  • meta: Stores metadata about the tape.
  • req: Request object. Used to match incoming requests against the tape.
  • res: Response object. The HTTP response that will be returned in case the tape matches a request.


  • url (String): Url relative to the host. E.g: /users
  • method (String): HTTP method. E.g: GET
  • headers (Object<String, String>): Request headers. E.g: {"content-type": "application/json", accept: "*/*"}
  • body (Buffer): Request body. E.g: Buffer.from("FOOBAR")


  • status (Number): HTTP response status code
  • headers (Object<String, [String]>): Response headers. E.g: {"content-type": ["application/json"]}
  • body (Buffer): Response body. E.g: Buffer.from("FOOBAR")

Request and Response body

If the content type of the request or response is considered human readable and uncompressed, the body will be saved in plain text.
Otherwise, the body will be saved as a Base64 string, allowing to save binary content.

Pretty Printing

If the request or response have a JSON content-type, their body will be pretty printed as an object in the tape for easier readability.
This means differences in formatting are ignored when comparing tapes, and any special formatting in the response will be lost.

File Name

New tapes will be created under the path directory with the name unnamed-n.json5, where n is the tape number.
Tapes can be renamed at will, for example to give some meaning to the scenario the tape represents.
If a custom tapeNameGenerator is provided, it will be called to produce an alternate file path under path that can be based on the tape contents. Note that the file extension .json5 will be appended automatically.

function nameGenerator(tapeNumber, tape) {
  // organize in folders by request method
  // e.g. tapes/GET/unnamed-1.json5
  //      tapes/GET/unnamed-3.json5
  //      tapes/POST/unnamed-2.json5
  return path.join(`${tape.req.method}`, `unnamed-${tapeNumber}`)

Recording Modes

Talkback proxying and recording behavior can be controlled through the record and fallbackMode options.

There are 3 possible recording modes:

Value Description
NEW If no tape matches the request, proxy it and save the response to a tape
OVERWRITE Always proxy the request and save the response to a tape, overwriting any existing one
DISABLED If a matching tape exists, return it. Otherwise, don't proxy the request and use fallbackMode for the response

The fallbackMode option lets you choose what to do when recording is DISABLED and an unknown request arrives.

There are 2 possible fallback modes:

Value Description
NOT_FOUND Log an error and return a 404 response
PROXY Proxy the request to host and return its response, but don't create a tape

It is recommended to DISABLE recording when using talkback for test running. This way, there are no side-effects and broken tests fail faster.

Both options accept either one of the possible modes to be used for all requests or a function that takes the request as a parameter and returns a valid mode.

const talkback = require("talkback")
const opts = {
  record: talkback.Options.RecordMode.DISABLED,
  fallbackMode: (req) => {
    if (req.url.includes("/mytest")) {
        return talkback.Options.FallbackMode.PROXY
      return talkback.Options.FallbackMode.NOT_FOUND

Custom request body matcher

By default, in order for a request to match against a saved tape, both request and tape need to have the exact same body.
There might be cases were this rule is too strict (for example, if your body contains time dependent bits) but enabling ignoreBody is too lax.

Talkback lets you pass a custom matching function as the bodyMatcher option.
The function will receive a saved tape and the current request, and it has to return whether they should be considered a match on their body.
Body matching is the last step when matching a tape. In order for this function to be called, everything else about the request should match the tape too (url, method, headers).
The bodyMatcher is not called if tape and request bodies are already the same.


function bodyMatcher(tape, req) {
    if (tape.meta.tag === "fake-post") {
      const tapeBody = JSON.parse(tape.req.body.toString());
      const reqBody = JSON.parse(req.body.toString());
      return tapeBody.username === reqBody.username;
    return false;

In this case we are adding our own tag property to the saved tape meta object. This way, we are only using the custom matching logic on some specific requests, and can even have different logic for different categories of requests.
Note that both the tape's and the request's bodies are Buffer objects.

Custom request URL matcher

Similar to the bodyMatcher, there's the urlMatcher option, which will let you customize how a request and a tape are matched on their URL.


function urlMatcher(tape, req) {
    if (tape.meta.tag === "user-info") {
      // Match if URL is of type /users/{username}
      return !!req.url.match(/\/users\/[a-zA-Z-0-9]+/);
    return false;

Custom request decorator

By default talkback will just proxy requests to the host as they are.
If you want to customize requests before they're proxied (or looked up in stored tapes) you can do so through the requestDecorator option.

requestDecorator takes an option that will receive the original request as a parameter and should return the modified request.

function requestDecorator(req) {
    delete req.headers['accept-encoding'];
    return req;

Custom response decorator

If you want to add a little bit of dynamism to the response coming from a matching existing tape or adjust the response that the proxied server returns, you can do so by using the responseDecorator option.
This can be useful for example if your response needs to contain an ID that gets sent on the request, or if your response has a time dependent field.

The function will receive a copy of the matching tape and the in-flight request object, and it has to return the modified tape. Note that since you're receiving a copy of the matching tape, modifications that you do to it won't persist between different requests.
Talkback will also update the Content-Length header if it was present in the original response.


We're going to hit an /auth endpoint, and update just the expiration field of the JSON response that was saved in the tape to be a day from now.

function responseDecorator(tape, req) {
  if (tape.meta.tag === "auth") {
    const tapeBody = JSON.parse(tape.res.body.toString())
    const expiration = new Date()
    expiration.setDate(expiration.getDate() + 1)
    const expirationEpoch = Math.floor(expiration.getTime() / 1000)
    tapeBody.expiration = expirationEpoch
    const newBody = JSON.stringify(tapeBody)
    tape.res.body = Buffer.from(newBody)
  return tape

In this example we are also adding our own tag property to the saved tape meta object. This way, we are only using the custom logic on some specific requests, and can even have different logic for different categories of requests.
Note that both the tape's and the request's bodies are Buffer objects and they should be kept as such.


By default talkback will try to reply to requests as fast as it can, but sometimes it's useful to understand how applications behave under real-world or even undesirably high response times.
Talkback lets you control response times both at a global or at a tape level.

The latency option will apply for all requests that match an existing tape or when using the PROXY fallback mode.

There are 3 possible types of values:

  • A number: Fixed number of milliseconds for all response times.
  • An array in the form [min, max]: Requests will take a random number of milliseconds in the given range.
  • A function (req) => latency: The function will be called for each request and it should return the desired number of milliseconds for the response time.

At the same time, tapes can define their own specific response times by adding a latency property to the meta object.
This property accepts both numbers and ranges and will take precedence over the global latency option.

  "meta": {
    "createdAt": "2017-09-10T23:19:27.010Z",
    "host": "http://localhost:8898",
    "resHumanReadable": true,
    "latency": [100, 500]

Error rate

Similar to what the latency option does, you might want to test how your application behaves when downstream services start failing.
Talkback can aid here through the errorRate option, by returning synthetic 503 errors back to you application.

The errorRate option will apply for all requests that match an existing tape or when using the PROXY fallback mode.

There are 2 possible types of values:

  • A number between 0 and 100 that defines the probabilty of returning an error for each request.
  • A function (req) => errorRate: The function will be called for each request and it should return the desired probabilty of error for that specific request.

At the same time, tapes can define their own specific error rates by adding an errorRate property to the meta object.

  "meta": {
    "createdAt": "2017-09-10T23:19:27.010Z",
    "host": "http://localhost:8898",
    "resHumanReadable": true,
    "errorRate": 50

Exit summary

If you are using talkback for your test suite, you will probably have tons of different tapes after some time. It can be difficult to know if all of them are still required.
To help, when talkback exits, it will print a list of all the tapes that have NOT been used and a list of all the new tapes. If your test suite is green, you can safely delete anything that hasn't been used.

===== SUMMARY (My Server) =====
New tapes:
- unnamed-4.json5
Unused tapes:
- not-valid-request.json5
- user-profile.json5

This can be disabled with the summary option.




npm i talkback

DownloadsWeekly Downloads






Unpacked Size

143 kB

Total Files


Last publish


  • avatar