Syntax basics

Everything you can write inside {{ }}.

Delimiters

An expression is anything between {{ and }}. The braces are literal:

Hello, {{ $trigger.body.name }}!

If the input field has no {{, Flero treats it as a plain string and skips evaluation entirely. So a field like Hello, world is just Hello, world.

Multiple expressions in one field

You can have any number of expression blocks in a single field. Each block is evaluated independently:

{{ $env.HOST }}/api/v{{ $node["Config"].json.version }}/users/{{ $trigger.body.id }}

If one block fails (typo in a node name), the others still resolve. The failed block is replaced with [EXPR-ERROR: …], not with empty string, that way the typo is visible in the output.

Whitespace inside a block

Whitespace inside {{ }} is ignored. These are equivalent:

{{ $node["HTTP"].json.body.email }}
{{$node["HTTP"].json.body.email}}
{{
  $node["HTTP"].json.body.email
}}

This is useful for long, hard-to-read expressions. Break them across lines for clarity.

Literal {{ and }}

If you need literal double-braces (rare, most often when generating templates for other template engines like Mustache or Jinja), wrap them in single braces or use a Code node to assemble the string programmatically. Flero does not currently provide an escape sequence inside {{ }} itself.

What an expression evaluates to

Expressions can resolve to any JSON value:

This matters when you build URLs, log messages, or any string context. If you need an array as an array (not a string), pass it through a field that accepts JSON directly, or use a Data Transform node to construct the surrounding structure.

Path navigation

After the root ($node["X"], $trigger, etc.), use dots for property access and [index] for array access:

$node["List users"].json.users[0].address.city
$trigger.body.items[2].sku
$workflow.metadata.tags[0]

Mixed mode is fine:

$node["Search"].json.results[0].highlights[2].text

If any intermediate value is null or missing, the whole expression evaluates to null (and renders as the string "null"). If you want a fallback, use a Code node, an If node, or a future-planned || operator, for now, the cleanest pattern is:

[Upstream] → [Data Transform: map { name: $input.name || "Anonymous" }] → ...

Array length

{{ $node["List"].json.items.length }}

.length works on arrays and strings.

String methods

A small set of string methods is available:

For richer string operations (regex, replace, substring, etc.) use a Text Processing node, it gives you the full kit with predictable behaviour.

Comparison and logical operators

Inside {{ }}, you can use:

• Arithmetic: +, -, *, /
• Comparison: ==, !=, <, >, <=, >=
• Logical: &&, ||

{{ $trigger.body.amount * 1.2 }}
{{ $trigger.body.amount > 1000 && $trigger.body.country == "US" }}

These are evaluated by the expression engine and the result is substituted as a value. For branching on the result, use an If node, which has a richer operator set.

Related

• Referencing data, the $ roots
• Functions, what's available beyond .toLowerCase(), etc.
• Debugging expressions

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