JS Runner

The JS Runner block runs JavaScript that you write. It receives the data from the previous block as input, and whatever you return goes to the next block. It is the most flexible block, for logic that no other block covers.

When to use it

• Do custom calculations or business rules, like prices, discounts or scoring.
• Combine several values, or reshape data in a way the Transformer field map can't.
• Read request details, call an API with httpClient, verify a JWT, or use any npm package you installed.
• Don't use it when a dedicated block exists. If Condition, Set Variable and the DB blocks are easier to read and show up clearly on the canvas.
• Don't use it to branch the flow. It has one Next handle. Compute the answer here and follow it with an If Condition or Switch.

Inputs

Field	Required	Default	What it does
Value	Yes	none	The JavaScript code. Modern syntax works.

Your code always runs async

The code runs inside an async function, so await works anywhere with no setup. const res = await httpClient.get(url); is fine. You don't need to mark anything async yourself.

Available as plain names in your code:

Name	What it is
input	The previous block's output.
outputs	Outputs saved with Save output to variable on other blocks, by name.
trigger	What started the run, and the events of a workflow run.
getRouteParam(k), getQueryParam(k), getHeader(k), getCookie(k), getRequestBody()	Read the request. In a workflow, getRequestBody() is the run's payload and the others return "".
setHeader(k, v), setCookie(name, options)	Add to the response.
getResponseBody(), getResponseStatus()	In an after middleware, the reply's body and status code. null anywhere else.
getConfig(key)	Read a value from App Config.
httpClient	Call other services.
abortSignal	Fires when the caller disconnects. Pass it to fetch or other slow work (details).
logger	logInfo, logWarn and logError.
jwt	Sign, verify and decode tokens. The only library that is built in.

There is no dayjs, _ (Lodash) or zod built in. To use a library, install it under Project Settings > npm Packages, then import it at the top of your code, for example import dayjs from "dayjs";. See Imports & Libraries.

Inside a custom block you also get params, the block's settings. The full list, with types, is in the JavaScript API Reference, and Where Your Code Runs shows which names exist where.

Outputs

Handle	When it is used
Next (right)	Always, when the code finishes. The next block receives the value you return.

If your code doesn't return anything, the next block gets an empty value.

Example

The previous block returned { "price": 100, "taxRate": 0.08 }. Code:

const total = input.price * (1 + input.taxRate);
return { total: total.toFixed(2) };

The next block receives:

{ "total": "108.00" }

More examples, like reading the request, calling an API and verifying a token, are on JS Runner examples.

How it behaves

• You must return a value. Without return, nothing is passed on.
• A thrown error fails the block. The Error Handler runs. To handle an error yourself, wrap the code in try and catch and return a result.
• Variables. A name you assign without const, let or var, like userId = 7;, is saved as a request variable that Get Variable and later blocks can read. Names declared with const or let stay inside this block.
• await always works. Every script is an async function, so you can await calls anywhere in it.
• No require(). Use import at the top of the code. A package that is not installed stops the workflow from compiling, with an error naming it.
• No time limit of its own. Keep long waits short. The whole route or workflow has a time limit.
• It replaces the flowing data with what you return.

Related blocks

• Transformer: reshape data without code.
• Set Variable: save a value without code.
• If Condition: branch on the result.
• HTTP Request: call an API without code.