Eval Step

The Eval step runs JavaScript inside the flow. You map values in as params, write code, and the value the code evaluates to becomes the step’s result. The runtime is small and embedded — not Node.js. There is no module system, no console, no timers and no network access. Everything the code works with must arrive through params.

Reach for Eval when a transformation needs logic. For a straight reshape of data you already have, the Map step is simpler and cannot fail at runtime.

The contract

Three rules cover nearly every mistake people make with this step:

Each params key becomes a global variable. A param named tickets is used in the code as tickets, not params.tickets and not input.tickets.
The result is the value of the last expression. There is no output variable to assign.
You cannot return at the top level. A bare return is a syntax error. To use return, wrap the code in a function and call it immediately.

// Fails: SyntaxError — Illegal return statement
return { total: 10 }

// Works: the function returns, and the call is the last expression
(function () {
  return { total: 10 }
})()

Configuration

The editor has a General tab (step name and ID) and a Code tab holding four sub-tabs:

Sub-tab	Purpose
Code	The JavaScript to run. Required.
Params	Mapped Params — a JSON object whose values are JSONPath expressions. Each key becomes a variable in the code.
Lookup	Sample flow data for testing only, and a Select Flow Run button that loads data from a previous run. Not saved with the step.
Examples	Ready-made snippets you can load into the editor.

Run Test executes the code against the Lookup data using the same engine the flow uses.

string

required

The JavaScript to evaluate.

JSON object

Variables to expose to the code. {"tickets": "$.GET_TICKETS.data", "threshold": 20} gives the code a tickets variable and a threshold variable. Values may be literals or JSONPath.

Examples

Every example below was run through the engine the step uses; the outputs are what it returned.

Reshape a list

Params

{ "tickets": "$.GET_TICKETS.data" }

Code

tickets
  .filter(function (t) { return t.resolved })
  .map(function (t) {
    return { ref: t.ref, team: t.team, hours: Math.round((t.minutes / 60) * 100) / 100 }
  })

With tickets =

[{"ref":"T-104","team":"support","minutes":45,"resolved":true}, {"ref":"T-105","team":"support","minutes":12,"resolved":false}, {"ref":"T-106","team":"onboarding","minutes":30,"resolved":true}]

[
  { "ref": "T-104", "team": "support", "hours": 0.75 },
  { "ref": "T-106", "team": "onboarding", "hours": 0.5 }
]

Aggregate with a wrapped function

Statements need a function around them, and the call must be the last expression.

(function () {
  var byTeam = {}
  tickets.forEach(function (t) {
    if (!byTeam[t.team]) byTeam[t.team] = { count: 0, minutes: 0 }
    byTeam[t.team].count += 1
    byTeam[t.team].minutes += t.minutes
  })
  var teams = Object.keys(byTeam)
  return {
    teams: teams.length,
    total_minutes: teams.reduce(function (sum, k) { return sum + byTeam[k].minutes }, 0),
    by_team: byTeam
  }
})()

{
  "teams": 2,
  "total_minutes": 87,
  "by_team": {
    "support": { "count": 2, "minutes": 57 },
    "onboarding": { "count": 1, "minutes": 30 }
  }
}

Guard against a param that resolved to nothing

A JSONPath that matches nothing resolves to an empty array — not null and not undefined — so check for that shape before working on it. Params

{ "tickets": "$.GET_TICKETS.data", "threshold": 20 }

Code

(function () {
  var list = Array.isArray(tickets) ? tickets : []
  if (list.length === 0) {
    return { status: 'no_data', slow: [] }
  }
  return {
    status: 'ok',
    slow: list.filter(function (t) { return t.minutes > threshold }).map(function (t) { return t.ref })
  }
})()

With no tickets: { "status": "no_data", "slow": [] }. With two tickets of 45 and 12 minutes: { "status": "ok", "slow": ["T-104"] }.

Dates and modern syntax

const, let, arrow functions, template literals, classes and spread all work. Params

{ "created_at": "$.trigger.created_at", "name": "$.trigger.name" }

Code

(function () {
  const created = new Date(created_at)
  const day = created.toISOString().slice(0, 10)
  return {
    day: day,
    weekday: ['Sun','Mon','Tue','Wed','Thu','Fri','Sat'][created.getUTCDay()],
    display: `${name.trim()} — ${day}`
  }
})()

With created_at = "2026-08-27T09:15:00Z" and name = " Dana Reyes ":

{ "day": "2026-08-27", "weekday": "Thu", "display": "Dana Reyes — 2026-08-27" }

What the runtime does not have

The following are absent. Code that uses them fails at run time, and the failing step ends the run.

require and import. There are no libraries to load — no Lodash, no Moment, no Math.js, no crypto. require('lodash') throws ReferenceError: require is not defined.
console. console.log throws ReferenceError: console is not defined. To inspect a value, make it part of the result and read it in the run’s logs.
Timers. setTimeout and setInterval are undefined. For a wait, use the Delay step.
Network and file access. For an outbound call, use the HTTP Request step.
An event loop. async functions and Promises never settle: an async function call returns an unsettled Promise, which is stored as {}. Write ordinary synchronous code.
A time limit. Nothing interrupts running code, so an unbounded loop hangs the run until it is cancelled. Bound every loop yourself.

Built-ins that are present include JSON, Math, Date, RegExp, Object, Array and String, with the common newer methods such as Object.entries and Array.prototype.includes. The editor summarises the level of support as core ES5 plus part of ES6.

Referencing the result

The value is stored flat under the step’s ID. If the step ID is eval_1 and the code returned an object:

$.eval_1.status
$.eval_1.slow[0]

If the code returned an array or a single value, $.eval_1 is that value. There is no .output or .result wrapper.

Troubleshooting

SyntaxError: Illegal return statement

The code has a return outside a function. Wrap the body in (function () { … })().

ReferenceError: <name> is not defined

Either the variable is a params key that was never added on the Params sub-tab, or it is something the runtime does not provide — require, console, setTimeout, fetch, process.

The result is an empty object

An async function was called, or a Promise was the last expression. Neither ever settles here. Rewrite the code synchronously.

A variable holds an empty array instead of the data

The JSONPath in Mapped Params matched nothing. Check the path against the Lookup tab, remembering that a step’s data is keyed by its step ID.

The run never finishes

A loop is not terminating. Nothing times the code out, so the guard has to be in the code.

Editing a flow changes its draft. Test there, then Publish to make the change take effect.

Next Steps

Map Step

Reshape data without writing code

Variable Mapping

JSONPath reference for Mapped Params

Condition Step

Branch on what the code returned

Functions Step

Built-in operations for common tasks

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Eval

Eval Step

The contract

Configuration

Examples

Reshape a list

Aggregate with a wrapped function

Guard against a param that resolved to nothing

Dates and modern syntax

What the runtime does not have

Referencing the result

Troubleshooting

Next Steps

Map Step

Variable Mapping

Condition Step

Functions Step

​Eval Step

​The contract

​Configuration

​Examples

​Reshape a list

​Aggregate with a wrapped function

​Guard against a param that resolved to nothing

​Dates and modern syntax

​What the runtime does not have

​Referencing the result

​Troubleshooting

​Next Steps

Map Step

Variable Mapping

Condition Step

Functions Step

Eval Step

The contract

Configuration

Examples

Reshape a list

Aggregate with a wrapped function

Guard against a param that resolved to nothing

Dates and modern syntax

What the runtime does not have

Referencing the result

Troubleshooting

Next Steps