# n8n: Items, JSON and Expressions, Explained by Doing > How n8n holds data: items, JSON and pinned test data, then expressions like {{ $json.client }} and a rounded calculation. Free hands-on beginner lesson. Source: https://learn.ynteractive.com/content/n8n-items-json-expressions-tutorial · Updated 2026-10-17 · Free from Stepthrough (https://learn.ynteractive.com) ## Practice it step by step [Start the free practice](https://learn.ynteractive.com/modules/n8n-data-and-expressions) [Take the free module: why it matters, practice and a quick check →](https://learn.ynteractive.com/courses/n8n-beginner/3) You do every step yourself in a practice copy of n8n. It checks your work as you go, and “Do it for me” finishes any step you get stuck on. Free, about 9 minutes, and nothing touches your real accounts. [Open full screen](https://learn.ynteractive.com/modules/n8n-data-and-expressions) Free · Live now · [n8n Automation Toolkit](https://learn.ynteractive.com/content/course/n8n-automation-toolkit) · [n8n](https://learn.ynteractive.com/content/tool/n8n) · ~9 min · 28 steps · Beginner Updated October 17, 2026. Built against the tool’s current screens and checked step by step. [How we check this lesson](https://learn.ynteractive.com/content/about#how-we-check) Items and expressions are where most [n8n](https://learn.ynteractive.com/content/tool/n8n) beginners get stuck. In this free lesson you pin five fake overdue [invoices](https://learn.ynteractive.com/content/glossary/invoice) on a [manual trigger](https://learn.ynteractive.com/content/glossary/manual-trigger) with set mock data, read them as Schema, Table and [JSON](https://learn.ynteractive.com/content/glossary/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](https://learn.ynteractive.com/modules/n8n-data-and-expressions) | ### The 4 parts, in short 1. Get some data in: Add the first step, Start it by hand, Open the trigger, Set mock data, and 2 more steps. 2. One item per record: Table view, JSON view, Close the trigger. 3. Expressions: Add a step, Search for Edit, Pick Edit Fields, Add a field, and 6 more steps. 4. 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](https://learn.ynteractive.com/content/n8n-tutorial-for-beginners-first-workflow) (module 1 of n8n Automation Toolkit) **In the course:** [Part 9 · n8n Automation Toolkit](https://learn.ynteractive.com/content/course/part-9-n8n-automation-toolkit) (Module 73 of the [full course](https://learn.ynteractive.com/content/course)) **Part of:** [n8n for Beginners: Your First Automations](https://learn.ynteractive.com/content/paths/n8n-beginner) (step 2 of 9). Next up: [n8n: Execute Step, Execute Workflow and Publish](https://learn.ynteractive.com/content/n8n-execute-step-execute-workflow-publish) ## 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](https://n8n.partnerlinks.io/2a7wq6ej1z0t) (affiliate link): the workflow tool this lesson teaches (free trial, no credit card). - [n8n docs: Expressions](https://docs.n8n.io/code/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”](https://learn.ynteractive.com/modules/n8n-data-and-expressions) ## 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 1. **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. 2. **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. 3. **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. 4. **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. 5. **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. ```text [{"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}] ``` 6. **Save it.** Click **Save**. ### 2. One item per record 7. **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. 8. **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. 9. **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 10. **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. 11. **Search for Edit.** Type **Edit**. 12. **Pick Edit Fields.** Click **Edit Fields (Set)**. 13. **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. 14. **Name it.** In **name**, type **reminder**. Name it reminder. 15. **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. 16. **Type a word.** At the end, type **is** (with a space each side). At the end, type is. 17. **Drag in the days.** Drag **days_late** to the end (here, click it). 18. **Finish the sentence.** Type **days late**. 19. **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 20. **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. 21. **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. 22. **Name it.** Type **late_fee**. Name it late fee, with an underscore. 23. **Change the type.** A fee is a number. Click the type box (it says **String**). A fee is a number. It says String. 24. **Number.** Choose **Number**. 25. **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. 26. **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. 27. **Run it.** Click **Execute step**. 28. **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. ## 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. [Start this module free](https://learn.ynteractive.com/modules/n8n-data-and-expressions) - Previous in n8n Automation Toolkit: [n8n: What Is n8n? Build Your First Workflow](https://learn.ynteractive.com/content/n8n-tutorial-for-beginners-first-workflow) - Next in n8n Automation Toolkit: [n8n: Run a Workflow on a Schedule (Every Monday at 8am)](https://learn.ynteractive.com/content/n8n-schedule-trigger-tutorial) ## More in n8n Automation Toolkit - [n8n: Send Form Answers Straight to Google Sheets](https://learn.ynteractive.com/content/n8n-form-to-google-sheets): Build a consultation request form in n8n (required fields, email, dropdown), test it, and add every answer as a new row in Google Sheets. No webhook, no form tool to… - [n8n: Summarize Customer Emails with OpenAI](https://learn.ynteractive.com/content/n8n-openai-summarize-emails): Your first AI step in n8n: send each customer email to OpenAI with one prompt and get a one-line summary plus the next step. No agent needed, about a cent of API use. - [Find and Use n8n Templates](https://learn.ynteractive.com/content/find-and-use-n8n-templates): Skip the blank canvas. Find a ready-made workflow in the n8n template gallery, import it into your workspace, connect Google Sheets and Gmail, test it with a real form… - [Litescrape in n8n: Swap SerpApi for a Cheaper Search API](https://learn.ynteractive.com/content/litescrape-n8n-serpapi-alternative): Switch a weekly n8n competitor-price workflow from SerpApi to Litescrape's Google Shopping API: change the HTTP Request URL, move the key into a Bearer Auth credential… - [n8n: Execute Step, Execute Workflow and Publish](https://learn.ynteractive.com/content/n8n-execute-step-execute-workflow-publish): Learn the buttons in today's n8n editor: run one node with Execute step, pin test data, run everything with Execute workflow, then Publish and copy the Production URL. - [Webhooks and Forms in n8n: Capture Data from Anywhere](https://learn.ynteractive.com/content/n8n-webhook-and-form-trigger-tutorial): Stop losing quote requests between inboxes. Build a form n8n hosts for you, catch entries from your own website with a webhook, and save every request to one Google… [All 18 modules in n8n Automation Toolkit](https://learn.ynteractive.com/content/course/n8n-automation-toolkit) ## More n8n tutorials - [n8n AI Agent Tutorial: Save Social Media Ideas to Google Sheets](https://learn.ynteractive.com/content/n8n-ai-agent-tutorial-google-sheets): Build an AI agent for BrightPath Marketing that generates social media post ideas and saves them to Google Sheets — chat trigger, AI Agent, system prompt, model… - [Lovable + n8n: Build a Web App with an AI Backend](https://learn.ynteractive.com/content/lovable-n8n-web-app-ai-backend): Build a contact form in Lovable and connect it to an n8n workflow — webhook trigger, AI Agent, Google Sheets, Gmail, and a Respond node that ties it all together. - [AI Receipt Extractor: Receipts to Google Sheets with n8n](https://learn.ynteractive.com/content/ai-receipt-extractor-n8n-google-sheets): Drop a photo of a receipt into an n8n chat. An AI agent reads it, pulls out the date, vendor and amount, picks a category, and adds a row to Google Sheets. - [n8n AI Email Responder: Draft Gmail Replies Automatically](https://learn.ynteractive.com/content/ai-email-responder-gmail-drafts-n8n): Build an n8n agent that watches your Gmail, reads each new customer email, and writes a reply as a draft in the same thread, ready for you to check and send. - [Daily Job Alerts: Email Each Tech Their Jobs from Google Sheets with n8n](https://learn.ynteractive.com/content/n8n-daily-job-alerts-google-sheets-gmail): Every morning at 7, n8n reads a plumbing company's jobs sheet, skips anything not assigned yet, and emails each job to the tech it's assigned to — customer, address… - [Gmail AI Triage: Auto-Label Every Email and Draft Replies](https://learn.ynteractive.com/content/gmail-ai-triage-labels-drafts-n8n): Build an n8n workflow that reads each new email, sorts it as a lead, a client or an invoice with a Text Classifier, adds the matching Gmail label, and drafts a reply to… ## Free cheat sheet Print it or save the PDF: the key steps and checks from this lesson on one page, free. - [n8n Beginner Cheat Sheet: Triggers, Items, Expressions](https://learn.ynteractive.com/content/cheat-sheets/n8n-beginner): 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. [All free printable cheat sheets](https://learn.ynteractive.com/content/cheat-sheets) ## Terms in this lesson Plain-English definitions, each with an example from a free lesson. [Invoice](https://learn.ynteractive.com/content/glossary/invoice) · [Manual trigger](https://learn.ynteractive.com/content/glossary/manual-trigger) · [JSON](https://learn.ynteractive.com/content/glossary/json) · [Late fee](https://learn.ynteractive.com/content/glossary/late-fee) · [Edit Fields (Set) node](https://learn.ynteractive.com/content/glossary/edit-fields-node) · [Execute workflow](https://learn.ynteractive.com/content/glossary/execute-workflow) [The full AI glossary](https://learn.ynteractive.com/content/glossary) ## Browse related tutorials [n8n Automation Toolkit (18)](https://learn.ynteractive.com/content/course/n8n-automation-toolkit) · [n8n tutorials (47)](https://learn.ynteractive.com/content/tool/n8n)