Question 1

What is an expression in n8n and what is its syntax?

Accepted Answer

Definition + syntax + the three references you use most is enough. "An expression fills a node parameter with a computed value instead of a fixed one. The syntax is {{ }} with JavaScript inside. The three references I use most: {{ $json.field }} — the current item's data {{ $('Node name').item.json.field }} — the previous node's item linked to the current one {{ $('Node name').first().json.field }} — that node's first item In practice I do not type them — I drag the field from the INPUT panel and n8n builds the expression. That is faster and removes path typos. One trap worth mentioning: the node name is inside the expression text, so renaming a node later breaks the references."

Question 2

What is the difference between $('Node').item and $('Node').first()?

Accepted Answer

This is the standard depth question on expressions. ".item returns the linked item — the one that gave rise to the current item. .first() always returns that node's first item, whatever the current item is. With one item in the flow the difference is invisible. With 100 items it is decisive: .item gives each item its own data, while .first() gives them all the same — the first item's — data. In practice this shows up as 100 emails going out all addressed to the same name. My rule: use .item when each item needs its own context, and .first() when you need one value that belongs to the whole run, such as a parameter from the trigger. In the Code node you use $('Node').itemMatching(i) instead of .item, because there n8n does not know on its own which item you are processing."

Question 3

Code that works in an expression stops working when moved to the Code node. Why?

Accepted Answer

This question checks whether you have read the documentation. "The reason is that they are not the same environment. The expression editor runs n8n's own expression engine; the Code node runs plain Node.js. Three concrete consequences: 1. Data transformation helpers — things like $if() and $jmespath() exist only in the expression editor, not the Code node. 2. $binary is not available in the Code node; binary data is reached another way. 3. Luxon methods. n8n adds some methods as its own extensions. For example, the expression version of plus()/minus() accepts (amount, unit), while native Luxon accepts only an object like { days: 7 }. That does not throw — it silently produces the wrong result, which makes it more dangerous. My first diagnostic step: on a ... is not a function error I check whether that method is marked as custom n8n functionality in the docs. If it is, I look for the native Luxon or JavaScript equivalent."

Expression	What it returns	Works in the Code node?
`$json`	The current item's JSON data	Yes — in "Run Once for Each Item" mode
`$input.item` / `$input.all()`	The current item / all incoming items	Yes
`$('Node').item`	That node's item linked to the current one	Yes (use `itemMatching()` to trace back)
`$('Node').first() / .last() / .all()`	That node's first / last / all items	Yes
`$binary`	The current item's binary data	No
`$now` / `$today`	The current moment / the start of today (Luxon objects)	Yes, though some n8n-specific methods differ
`$jmespath()`, `$if()`	Data transformation helpers	No — expression editor only

Expressions: dynamic values

📚 Sources and documentation