Sheet Generation API: Structured JSON In, Formatted Spreadsheet Out

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Iteration La · 2026-04-30 · via DEV Community

<h2> Spreadsheet Generation Is Still Painful </h2> Every application eventually needs to export data as a spreadsheet. Financial reports, inventory lists, invoices, analytics dashboards — at some point, someone asks for an Excel file and you start searching for libraries. If you work in JavaScript, you find ExcelJS or SheetJS. Python has openpyxl. Ruby has caxlsx. Java has Apache POI. Each one has a different API surface, different quirks, and different failure modes. The common thread: they're all verbose. Generating a simple XLSX with headers, currency formatting, and a SUM formula takes dozens of lines of boilerplate — cell references, style objects, merge ranges, format strings. CSV is simpler but limited. No formatting, no formulas, no multiple sheets. You get a flat text file and hope the recipient's spreadsheet app guesses the delimiter correctly. And if you need both formats? You maintain two separate code paths. One for XLSX with its style objects and cell references. One for CSV with its string concatenation. Both doing the same conceptual thing — turning rows of data into a spreadsheet — in completely different ways. <h2> One API Call, Three Formats </h2> The <a href="https://iterationlayer.com/products/sheet-generation" rel="noopener noreferrer">Iteration Layer's Sheet Generation API</a> takes a single JSON payload describing your spreadsheet and returns a formatted file. The same payload produces XLSX, CSV, or Markdown — you just change the <code>format</code> field. Here's the simplest possible request: <div class="highlight js-code-highlight"> <pre class="highlight typescript"><code>import { IterationLayer } from "iterationlayer"; const client = new IterationLayer({ apiKey: "YOUR_API_KEY", }); const result = await client.generateSheet({ format: "xlsx", sheets: [ { name: "Q1 Revenue", columns: [ { name: "Company", }, { name: "Region", }, { name: "Revenue", }, ], rows: [ [ { value: "Acme Corp", }, { value: "North America", }, { value: 48500.00, format: "currency", currency_code: "USD", }, ], [ { value: "Globex Ltd", }, { value: "Europe", }, { value: 37200.00, format: "currency", currency_code: "EUR", }, ], ], }, ], }); </code></pre> </div> <div class="highlight js-code-highlight"> <pre class="highlight json"><code>{ "success": true, "data": { "buffer": "UEsDBBQAAAAIAA...", "mime_type": "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet" } } </code></pre> </div> The response contains the file as a base64-encoded buffer with the appropriate MIME type. Decode it, save it, send it to a client — the file is ready to open in Excel, Google Sheets, or Numbers. <h2> The Data Model </h2> The API uses a positional model. You define columns, then provide rows as ordered arrays of cells. Each cell has a value and optional formatting. Columns define the header row. Each column has a <code>name</code> and an optional <code>width</code>: <div class="highlight js-code-highlight"> <pre class="highlight json"><code>{ "columns": [ { "name": "Product", "width": 30 }, { "name": "Units Sold" }, { "name": "Unit Price" }, { "name": "Total" } ] } </code></pre> </div> Cells carry the data. At minimum, a cell has a <code>value</code>. Add <code>format</code> to control how it's displayed: <div class="highlight js-code-highlight"> <pre class="highlight json"><code>{ "value": 1500.50, "format": "currency", "currency_code": "USD" } </code></pre> </div> Supported cell formats: <code>text</code>, <code>number</code>, <code>decimal</code>, <code>currency</code>, <code>percentage</code>, <code>date</code>, <code>datetime</code>, <code>time</code>, and <code>custom</code>. Number styles control how digits are grouped. Four styles cover the major international conventions: <ul> <li> <code>comma_period</code> — 1,234.56 (US, UK, most of Asia)</li> <li> <code>period_comma</code> — 1.234,56 (Germany, Brazil, most of Europe)</li> <li> <code>space_comma</code> — 1 234,56 (France, Norway, Sweden)</li> <li> <code>space_period</code> — 1 234.56 (Estonia, South Africa)</li> </ul> Date styles control how date, datetime, and time values are formatted. Pass any Excel date format code — <code>dd/mm/yyyy</code> for European dates, <code>mm/dd/yyyy</code> for US, <code>d mmmm yyyy</code> for full month names, <code>hh:mm</code> for short times. The format code is applied natively in XLSX and rendered to the equivalent string in CSV and Markdown. Currency codes follow ISO 4217. The API supports 162 codes — from USD and EUR to THB and CLP — and maps each one to its correct symbol. <h2> Styles: Base and Per-Cell </h2> Instead of styling every cell individually, you define base styles for headers and body rows, then override specific cells as needed. <div class="highlight js-code-highlight"> <pre class="highlight typescript"><code>import { IterationLayer } from "iterationlayer"; const client = new IterationLayer({ apiKey: "YOUR_API_KEY", }); const result = await client.generateSheet({ format: "xlsx", styles: { header: { is_bold: true, font_size_in_pt: 12, background_color: "#1a1a2e", font_color: "#ffffff", horizontal_alignment: "center", }, body: { font_size_in_pt: 10, }, }, sheets: [ { name: "Styled Report", columns: [ { name: "Item", }, { name: "Amount", }, ], rows: [ [ { value: "Server costs", }, { value: 4200.00, format: "currency", currency_code: "USD", styles: { font_color: "#cc0000", }, }, ], ], }, ], }); </code></pre> </div> <div class="highlight js-code-highlight"> <pre class="highlight json"><code>{ "success": true, "data": { "buffer": "UEsDBBQAAAAIAA...", "mime_type": "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet" } } </code></pre> </div> The <code>styles.header</code> and <code>styles.body</code> objects set defaults. The <code>styles</code> field on individual cells overrides them. Available style properties include <code>font_family</code>, <code>font_size_in_pt</code>, <code>is_bold</code>, <code>is_italic</code>, <code>font_color</code>, <code>background_color</code>, <code>horizontal_alignment</code>, and <code>number_format</code>. For XLSX output, you can also embed custom fonts by including a <code>fonts</code> array with base64-encoded font buffers. <h2> Formulas </h2> Cells whose value starts with <code>=</code> are treated as formulas. In XLSX output, they're written as native Excel formulas — Excel evaluates them when the file opens. In CSV and Markdown output, the API evaluates them server-side and writes the computed result. The server-side evaluator supports <code>SUM</code>, <code>AVERAGE</code>, <code>COUNT</code>, <code>MIN</code>, and <code>MAX</code> on cell ranges. <div class="highlight js-code-highlight"> <pre class="highlight json"><code>{ "sheets": [ { "name": "Invoice", "columns": [ { "name": "Description" }, { "name": "Amount" } ], "rows": [ [ { "value": "Design work" }, { "value": 3200, "format": "currency", "currency_code": "USD" } ], [ { "value": "Development" }, { "value": 8500, "format": "currency", "currency_code": "USD" } ], [ { "value": "QA testing" }, { "value": 1500, "format": "currency", "currency_code": "USD" } ], [ { "value": "Total" }, { "value": "=SUM(B2:B4)", "format": "currency", "currency_code": "USD" } ] ] } ] } </code></pre> </div> In XLSX, the Total cell contains <code>=SUM(B2:B4)</code> and Excel computes it. In CSV, the API evaluates the formula and writes <code>13200</code>. Same input, correct output in every format. <h2> Cell Merging </h2> Cells can span multiple columns and rows using <code>from_col</code>, <code>to_col</code>, <code>from_row</code>, and <code>to_row</code>. This works in XLSX output for building headers that span columns or summary rows that stretch across a table. <div class="highlight js-code-highlight"> <pre class="highlight json"><code>{ "value": "Q1 2026 Financial Summary", "from_col": 0, "to_col": 3, "styles": { "is_bold": true, "horizontal_alignment": "center", "font_size_in_pt": 14 } } </code></pre> </div> <h2> The Pipeline: Extract Then Generate </h2> The Sheet Generation API is designed to compose with the Document Extraction API. A common workflow: you receive invoices as PDFs, extract the line items, and generate a consolidated spreadsheet. <div class="highlight js-code-highlight"> <pre class="highlight typescript"><code>import { IterationLayer } from "iterationlayer"; const client = new IterationLayer({ apiKey: "YOUR_API_KEY", }); // Step 1: Extract invoice data const extraction = await client.extractDocument({ file: invoicePdfBase64, schema: { vendor: { type: "text", }, invoice_number: { type: "text", }, line_items: { type: "array", items: { description: { type: "text", }, quantity: { type: "number", }, unit_price: { type: "currency", }, total: { type: "currency", }, }, }, }, }); const fields = extraction.data.fields; // Step 2: Generate the spreadsheet const rows = fields.line_items.value.map((item) => [ { value: item.description.value, }, { value: item.quantity.value, format: "number", }, { value: item.unit_price.value, format: "currency", currency_code: "USD", }, { value: item.total.value, format: "currency", currency_code: "USD", }, ]); const sheet = await client.generateSheet({ format: "xlsx", styles: { header: { is_bold: true, background_color: "#f0f0f0", }, }, sheets: [ { name: `Invoice ${fields.invoice_number.value}`, columns: [ { name: "Description", }, { name: "Quantity", }, { name: "Unit Price", }, { name: "Total", }, ], rows, }, ], }); </code></pre> </div> <div class="highlight js-code-highlight"> <pre class="highlight json"><code>{ "success": true, "data": { "buffer": "UEsDBBQAAAAIAA...", "mime_type": "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet" } } </code></pre> </div> PDF in, formatted XLSX out. Two API calls, no local dependencies. <h2> Get Started </h2> The Sheet Generation API is available now. Check the <a href="https://iterationlayer.com/products/sheet-generation" rel="noopener noreferrer">docs</a> for the full API reference, including all cell formats, style properties, and formula support. If you're already using the Document Extraction API, the two compose naturally — extraction output maps directly to sheet rows. If you're starting fresh, the <a href="https://iterationlayer.com/products/sheet-generation" rel="noopener noreferrer">Sheet Generation quickstart</a> is the fastest path to your first generated spreadsheet. <a href="https://platform.iterationlayer.com/signup" rel="noopener noreferrer">Sign up</a> for a free account — no credit card required — and generate your first spreadsheet in minutes.

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