What is an item in n8n?

One record, such as one invoice or one form submission, with named fields. Nodes pass lists of items to each other and usually run once per item.

What does $json mean in an n8n expression?

$json is the current item's data, so {{ $json.client }} is this item's client field.

How do I test n8n without connecting an app?

Open the trigger, click set mock data, paste JSON and save it. Test runs use the pinned data.

Why does my n8n number have so many decimals?

Computers store decimals in binary, so 760 * 0.02 becomes 15.200000000000001. Add .round(2) to the expression.

n8n Expressions and Items Tutorial: Drag, Map, Calculate

Free · Live nown8n Automation Toolkit n8n~9 min28 stepsBeginner

Updated October 17, 2026. Built against the tool’s current screens and checked step by step. How we check this lesson

Items and expressions are where most n8n beginners get stuck. In this free lesson you pin five fake overdue invoices on a manual trigger with set mock data, read them as Schema, Table and JSON, and then write one reminder line, one email field and one rounded late fee that work for every invoice, all in a practice copy of the n8n editor. You also learn the two gotchas: a Number field mangles a typed formula until you switch it to Expression, and 760 * 0.02 shows as 15.200000000000001 until you add .round(2).

At a glance

Question	Answer
What you build	See how n8n holds data: pin five fake invoices, read them as items, Table and JSON, then write one reminder per invoice and a late fee with drag-and-drop expressions.
Tools	n8n
Time	About 9 minutes, 28 steps in 4 parts
Level	Beginner, no coding needed
Cost	Free
Ways to learn it	Practice it step by step

The 4 parts, in short

Get some data in: Add the first step, Start it by hand, Open the trigger, Set mock data, and 2 more steps.
One item per record: Table view, JSON view, Close the trigger.
Expressions: Add a step, Search for Edit, Pick Edit Fields, Add a field, and 6 more steps.
Drop, calculate, run: Drop a whole field, Add one more field, Name it, Change the type, and 5 more steps.

Start here first: n8n: What Is n8n? Build Your First Workflow (module 1 of n8n Automation Toolkit)

In the course: Part 9 · n8n Automation Toolkit (Module 72 of the full course)

Part of: n8n for Beginners: Your First Automations (step 2 of 9). Next up: n8n: Execute Step, Execute Workflow and Publish

n8n manual trigger node with the EDIT OUTPUT box open for set mock data, showing the example JSON and Cancel and Save buttons

The five invoices (fake)

Paste these into set mock data to follow along in your own n8n. The email addresses use .example domains.

client	invoice	amount	days_late
Lakeside Dental	INV-1042	480	12
Northpoint Realty	INV-1045	1250	34
Brightline Studio	INV-1047	300	5
Cedar & Vine Law	INV-1051	2100	48
Harbor Lane Marketing	INV-1053	760	19

Items: one per record

n8n passes a list of items between nodes. Each item is one record with named fields, and most nodes run once per item: 5 invoices in, 5 results out. Switch the OUTPUT between Schema, Table and JSON to read the same items three ways.

Pinned test data

Open a trigger and click set mock data, paste a JSON list and click Save. The banner says This data is pinned for test executions, and test runs reuse it instead of calling the real app.

Expressions by dragging

In Edit Fields (Set), click Add Field and drag a field from INPUT onto the value: n8n writes {{ $json.client }}, this item's client. The Result line shows Item 0; click › to preview the next item. Drag a field onto Drag input fields here to create a whole new field in one go.

Calculations and types

Anything inside {{ }} is JavaScript, so {{ ($json.amount * 0.02).round(2) }} calculates a 2% late fee rounded to cents. If the field type is Number, switch the value to Expression before typing, or the field strips everything that is not a digit.

Links

n8n (affiliate link): the workflow tool this lesson teaches (free trial, no credit card).
n8n docs: Expressions: the full expression reference.

Disclosure: links marked "affiliate link" are partner links. If you sign up through one, Stepthrough may earn a commission at no extra cost to you. The lesson is free either way and does not depend on it.

Try it yourself, free

Reading the steps is a start. Doing them is how it sticks. The interactive module walks you through every click in a practice copy of n8n, checks each step, and never touches your real accounts.

Start “n8n: Items, JSON and Expressions, Explained by Doing”

Step by step: n8n: Items, JSON and Expressions, Explained by Doing

All 28 steps of the interactive module, in order. In the module you watch each one, then do it yourself in a practice copy that checks your work.

1. Get some data in

Add the first step. Clearview Bookkeeping has five overdue invoices and writes every reminder by hand. Before you automate that, you need to see how n8n holds data. This is an empty workflow. Click Add first step....
Clearview Bookkeeping has five overdue invoices, and writes every reminder by hand. Before you automate that, you need to see how n8n holds data.
Start it by hand. Click Trigger manually. While you learn, a button is the easiest start.
While you learn, a button is the easiest way to start.
Open the trigger. The trigger is on the canvas as When clicking 'Execute workflow'. Open it (double-click; here, click).
The trigger sits on the canvas.
Set mock data. It has no parameters, and the OUTPUT says No trigger output. Instead of connecting a real system, give it test data: click set mock data.
It has no settings, and no output yet. Instead of connecting a real system, give it some test data.

Paste the invoices. The EDIT OUTPUT box holds an example. Replace it with the five invoices: a list in square brackets, one pair of curly brackets per invoice.

The edit output box holds an example. Replace it with the five invoices. It's JSON. A list in square brackets, with one set of curly brackets for each invoice.

[{"client":"Lakeside Dental","email":"billing@lakeside-dental.example","invoice":"INV-1042","amount":480,"days_late":12},{"client":"Northpoint Realty","email":"ap@northpoint.example","invoice":"INV-1045","amount":1250,"days_late":34},{"client":"Brightline Studio","email":"hello@brightline.example","invoice":"INV-1047","amount":300,"days_late":5},{"client":"Cedar & Vine Law","email":"office@cedarvine.example","invoice":"INV-1051","amount":2100,"days_late":48},{"client":"Harbor Lane Marketing","email":"finance@harborlane.example","invoice":"INV-1053","amount":760,"days_late":19}]

Save it. Click Save.

2. One item per record

Table view. A purple banner says This data is pinned for test executions, and the OUTPUT says 5 items. Schema shows one item's fields. Click Table.
A banner says, this data is pinned for test executions. The output says five items. One item for each invoice. Schema shows the fields of one item.
JSON view. Five rows, one per invoice. Text fields have a T, numbers a #. Click JSON to see the raw data.
Five rows. One per invoice.
Close the trigger. Same data, written as JSON. Every app n8n talks to sends data like this. Close the node.
Same data, written as JSON. Almost every app sends data to n8n like this. Close the node.

3. Expressions

Add a step. A pin on the trigger means it uses the pinned data. Click the + after it.
The pin on the trigger means it's using your test data.
Search for Edit. Type Edit.
Pick Edit Fields. Click Edit Fields (Set).
Add a field. The INPUT shows 5 items and the fields of the first one. Click Add Field.
The input shows five items, and the fields of the first one.
Name it. In name, type reminder.
Name it reminder.
Drag in the client. Drag client onto value (here, click it). n8n writes {{ $json.client }}: *this item's client*.
Here, click it. n8n writes, dollar json dot client. It means, this item's client.
Type a word. At the end, type is (with a space each side).
At the end, type is.
Drag in the days. Drag days_late to the end (here, click it).
Finish the sentence. Type days late.
Check the next item. The Result shows Lakeside Dental is 12 days late, for Item 0. Click › next to it to preview item 1.
The result shows Lakeside Dental is twelve days late. That's item zero.

4. Drop, calculate, run

Drop a whole field. Item 1 reads Northpoint Realty is 34 days late: one expression, a different answer per item. Now drag email onto Drag input fields here (here, click it).
Northpoint Realty is thirty four days late. One expression, a different answer for every item. Here, click it.
Add one more field. n8n made a field named email with {{ $json.email }} in one go. Click Add Field for a late fee.
n8n made a whole field, called email, in one go. Now a late fee.
Name it. Type late_fee.
Name it late fee, with an underscore.
Change the type. A fee is a number. Click the type box (it says String).
A fee is a number. It says String.
Number. Choose Number.
Switch to Expression. A Number box only takes digits, so a typed {{ }} gets mangled. Point at the value and click Expression.
Careful here. A number box only takes digits. If you type curly brackets into it, they get mangled. Point at the value, and click Expression first.
Calculate the fee. Paste the formula: 2% of the amount, rounded to cents. Without .round(2), 760 × 0.02 shows as 15.200000000000001.
Two percent of the amount, rounded to cents. Without the round, seven sixty times point oh two shows up as fifteen point two, with a long tail of zeros and a one.
Run it. Click Execute step.
Five in, five out. 5 items in, 5 items out: each invoice got its own reminder, email and late fee. Close the node.
Five items in. Five items out. Each invoice got its own reminder, email and late fee, from one node. Close the node.

Screenshots

n8n trigger OUTPUT in Table view with the banner This data is pinned for test executions and 5 items: client, email, invoice, amount and days_late for five fake invoices

The same five fake invoices in n8n's JSON view, a list in square brackets with one set of curly brackets per invoice

n8n Edit Fields with a reminder field built from dragged expressions, with the result Lakeside Dental, invoice INV-1042 for $480 is 12 days late.

n8n Edit Fields with reminder, email and a Number field late_fee set to {{ ($json.amount * 0.02).round(2) }}, and an OUTPUT table of 5 items with fees 9.6, 25, 6, 42 and 15.2

Frequently asked questions

What is an item in n8n?: One record, such as one invoice or one form submission, with named fields. Nodes pass lists of items to each other and usually run once per item.
What does $json mean in an n8n expression?: $json is the current item's data, so {{ $json.client }} is this item's client field.
How do I test n8n without connecting an app?: Open the trigger, click set mock data, paste JSON and save it. Test runs use the pinned data.
Why does my n8n number have so many decimals?: Computers store decimals in binary, so 760 * 0.02 becomes 15.200000000000001. Add .round(2) to the expression.

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A free one-page n8n beginner cheat sheet: what n8n is, triggers vs actions, items and JSON, expressions, test vs production, schedules, forms and costs.

Tool cheat sheet Free PDF

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Plain-English definitions, each with an example from a free lesson.

Invoice Manual trigger JSON Late fee Edit Fields (Set) node Execute workflow

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