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    Try backdoorthen on RunKit

    Backdoor 🚪

    Promises to make working with Promises great again 😱 either calling a given thenable, or bypassing it entirely in favour of returning mocked data instead.

    It's especially convenient for working on functions that are lower in the promise chain.


    Say you're working on this piece of code:

    const formData = {
      firstName: 'Bob',
      familyName: 'Loblaw'

    You've already refined your createUser promise to perfection, now your focus is on doMoreAsyncThings.

    You know you'll have to run that one a few times before getting it right, and you don't want to actually create a new user every time you do.

    Still, you have a demo scheduled with your PM tomorrow, and you want to be able to show that createUser actually works.

    While switching back and forth between different feature branches is an option, it's kinda nice if you can avoid it.

    Backdoor allows you to easily say: "If the firstName is 'backdoor', don't actually create a user named 'backdoor' and just return a hardcoded { username: bobLoblaw } object. Anything else, actually do create a user with the input data".

    Here's how it could look like:

    const createUserWithBackdoor = backdoor({
      actualThenable: createUser,
      input: formData.firstName,
      resolvedValue: { userName: 'bobLoblaw' },
      rejectedValue: { error: 'kaboom' }


    npm i -D backdoorthen
    # yes, the name "backdoor" was already taken...


    const backdoor = require('backdoorthen');


    import backdoor from 'backdoorthen';

    Default behaviour

    Although flexible, backdoor is built with string inputs in mind.

    INPUT                    PROMISE
    "backdoor"            -> resolves slow (5s)
    "backdoor-fast"       -> resolves fast (1s)
    "backdoor-error"      -> rejects slow  (5s)
    "backdoor-error-fast" -> rejects fast  (1s)

    Override logic

    backdoor's params object accepts an object on its config key.


    You can control the delay it takes the promise to return the mocked data via the and the config.slow properties:

    const backdooredProm = backdoor({
      // ...omitted for brevity,
      config: {
        fast: 800, // ms
        slow: 8000, // ms


    If you'd rather type backdoor+fast instead of backdoor-fast to make the fake promise resolve quickly, you can define the separator character like so:

    const backdooredProm = backdoor({
      // ...omitted for brevity,
      config: { separator: '+' }


    And finally, if your use-case does not rely on strings or if you'd rather implement your own logic, you can define an assessor function that must have the following signature:

    const assessor = input => ({
      isBackdoor: boolean,
      doResolve: boolean,
      isFast: boolean

    which you would then pass as a property of the config object:

    const backdooredProm = backdoor({
      // ...omitted for brevity,
      config: { assessor: yourTailoredAssessor }


    • true: skip the actual promise and return mocked data
    • false: return the actual promise


    • true: the fake promise will resolve with the mocked response/result
    • false: the fake promise will reject with the mocked error

    isFast: wait x ms before fulfilling the promise

    • true: wait 5000ms by default (override with
    • false: wait 1000ms by default (override with config.slow)

    (!) Note:

    • For readability's sake this readme uses the words promise and thenable interchangeably.
    • For accuracy's sake: promise here really is a function that returns a promise.

    Recommended use

    Create a withBackdoor function that takes the original thenable as its argument:

    const withBackdoor = thenable => backdoor({
      actualThenable: thenable,
      input: formData.firstName,
      resolvedValue: { userName: 'bobLoblaw' },
      rejectedValue: { error: 'kaboom' }

    This allows you to do:


    which is so close to what your code would look like if you hadn't backdoored your promise.

    That makes it very easy to remove backdoor once you're ready for your final commit:


    Try it live, no need to commit 😉

    Click that >_ Try on RunKit button 👉


    npm i backdoorthen

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    • johanfive