Dynamic values and js: expressions in block inputs

Read this before you fill any block field with a value that changes per request, such as an id from the input, a query parameter or a built string. It answers "how do I make this value dynamic".

The rule for a dynamic value

A dynamic value is one that is worked out per request instead of typed in.

A block text input is a literal unless it starts with js:.

• No prefix: the text is saved and used as it is. input.id is the nine characters input.id, not the id.
• Prefix js:: the rest is JavaScript, run at request time. Its result is the value.

{ "key": "userId", "value": "js: return input.userId" }

There is no other way to mix text and data. templates do not exist.

Write the code after js:

After the prefix, write the body of a function and return the value.

• js: return input.userId gives the user id.
• A bare expression without return gives undefined. js: input.userId is wrong.
• await works, so js: return await httpClient.get(url) is fine.
• Several statements are fine. Keep short values on one line.
• Nothing is evaluated unless the text starts with exactly js:. A space before it, or a different case, makes it a literal.
• Never wrap a value in . It is saved as plain text.

What you can read

Name	What it is
input	The output of the block just before this one. get_block_schemas with blockTypes shows what each block outputs, e.g. after Get Request Body it is the body itself, so read input.email, not input.body.email.
outputs.name	A block output saved with saveAsVariable. See Save output to variable.
trigger	What started the run. In a workflow, trigger.data is the list of events.
params.name	A custom block's own settings. Only inside a custom block.
getQueryParam("k"), getRouteParam("k"), getHeader("k"), getCookie("k")	Read the request. Each gives "" when the value is missing.
getRequestBody()	The parsed request body.
httpRequestMethod, httpRequestRoute	The method and the path of the request.
getConfig("KEY")	An app config value. Use it for keys and secrets.
jwt.sign, jwt.verify, jwt.decode	Token helpers.
logger.logInfo(...)	Write a log entry.
httpClient.get(url)	Call an outside API.
a variable you set	A variable made by a Set Variable block, read by its bare name, such as userId.
Math, JSON, Date, String and the other standard JavaScript names	Always there.

Names that do not exist: vars, cfg and data. js: return vars.x and js: return cfg.KEY fail. Read a variable by its name and an app config value with getConfig("KEY").

cfg:KEY is a different thing. It is the prefix an integration config uses to point at an app config key. It never works in a block field. See App config reference.

The full list of helpers is in Scripting Context.

Which fields accept js:

Most text inputs that hold a value accept it:

Where	Fields
Set Variable, Array Operations	value
Console Log, Cloud Logs	message
Response	httpCode, for a status that depends on the flow, such as js: return input.created ? 201 : 200;
HTTP Request	url, header names and values, the values inside body
Get and Set Header, Cookie, Param	name, value, and for cookies domain, path, expiry
Database blocks	connection, tableName, condition attribute and value, limit, offset, after, sort attribute, and the values inside value for Insert and Update
KV Operations	connection, key, value, ttl
If, Switch	condition sides and case values
For Loop	start, end, step
Send Message	connection, destination, key, header and option values, payload values
Trigger Workflow	runAt, scheduleId, the values inside data
Custom block	any text parameter

A value inside an object or a list counts. { "headers": { "X-User": "js: return input.id" } } works.

If you are unsure about a field, call get_block_schemas for the block. A field that takes js: says so in its description. A field that does not, such as a name chosen from a fixed list, treats js: as a literal.

Fields that are already code

These are already code. Never add js: to them. Write plain JavaScript that returns the value. A leading js: is ignored and the save warns about it:

• JS Runner: value.
• Transformer: js, when useJs is on.
• Response: transformScript, when transformEnabled is on.
• DB Native, KV Raw Connection and Send Message in raw mode: js.
• A custom block's code.

A Transformer fieldMap is also plain text. It maps a source key to a destination key and never takes js:.

{ "value": "return { id: input.id, total: input.price * input.qty };" }

Raw SQL conditions on database blocks are the one place is valid, for SQL parameters. See Database conditions. Everywhere else it is a mistake.

What code can see

The same names reach a js: text input, a code field and a custom block's code. Code runs as the body of an async function, so await, const, let and arrow functions all work. const x is local to that code.

Name	Where
input	Everywhere. The previous block's output.
params	Only inside a custom block. Its settings.
outputs.name	After a block with saveAsVariable ran in this request. Before that outputs is undefined.
trigger	Everywhere: trigger.kind, trigger.source and trigger.data (a list of events).
logger.logInfo, logger.logWarn, logger.logError	Everywhere.
httpClient.get, post, put, delete, patch	Everywhere. Each returns the response, with the body in .data.
dbQuery(sql, params)	DB Native block on PostgreSQL or MySQL. Placeholders are $1, $2; MySQL also takes ?.
db, ObjectId	DB Native block on MongoDB.
getResponseBody(), getResponseStatus()	After middlewares only. null anywhere else.
setHeader(k, v), setCookie(name, options)	Routes. They set a header or cookie on the reply.

There is no query, body or params.id for the request. Read the request with getQueryParam("k"), getRouteParam("k"), getHeader("k") and getRequestBody(). Each gives "" when a value is missing; getRequestBody() gives null with no body.

To pass a value to a later block, either:

• save the block output with saveAsVariable, then read outputs.name;
• add a Set Variable block and read the variable by its bare name;
• assign a bare name in code, such as userId = 5, and read userId later. Do not declare it with const or let: that makes it local.

Everything is per request. Nothing is shared between requests.

A leading js: in a code field is stripped when the canvas compiles, and the save warns about it. It never makes const fail: const and let work in code.

Build a string

Use a template string with ${ } inside the js: code:

{ "message": "js: return `Hello ${input.name}`" }

For a value from the request:

{ "message": "js: return `Hello ${getQueryParam('name')}`" }

Fixed text needs no prefix: "message": "Order created".

Right and wrong

Want	Wrong	Right
The id from the previous block	a template around input.id	js: return input.id
The id from the previous block	input.id	js: return input.id
The id from the previous block	js: input.id	js: return input.id
Hello and a name	Hello plus a template	js: return `Hello ${input.name}`
A query parameter	getQueryParam('page')	js: return getQueryParam('page')
An app config value	js: return cfg.API_KEY	js: return getConfig('API_KEY')
A variable set earlier	js: return vars.userId	js: return userId
A status code	js: input.ok ? 200 : 400	js: return input.ok ? 200 : 400
Fixed text	js: return "Order created"	Order created

The templates people reach for, which never work:

{{ input.id }}
Hello {{ input.name }}

Common mistakes

Mistake	What happens	Fix
No js: prefix	The field is the literal text, and the route answers with it.	Start the value with js:.
js: without return	The value is undefined.	Add return.
template	Saved as plain text.	Use js: with a template string.
js: in a code field	The field is already code, so the prefix is not needed there.	Remove the prefix and write plain JavaScript.
vars., cfg. or data.	These names do not exist, and the run fails.	See What you can read.

edit_canvas with validate: true returns a warning for a field that holds a template or starts with input., vars., cfg. or data. but has no js:. The warning does not stop the save, so read it.

Save a block output

Any block can keep its output for later blocks with saveAsVariable: { "enabled": true, "name": "user" } in its data. A later js: input reads js: return outputs.user.id and code reads outputs.user.id. Name rules and when to use it are in Save output to variable.

Related pages

• Scripting Context: every name that code can read.
• Custom block reference: parameters that accept js:.
• Canvas guide: how a canvas runs, handles and edit_canvas.
• Route reference: the settings of a route.