Document Template Designer Web App

A) Web app (template designer UI)

Use an embedded DOCX editor:

ONLYOFFICE Docs (most common for embedding)

Flow:

Your UI has a page /templates/:id/edit

You embed ONLYOFFICE editor in that page (iframe + their JS config)

Add a custom button: Insert Placeholder

Clicking it inserts something like {{Customer.Name}} at the cursor

B) Backend services

You typically have 3 backend parts:

File service

Store original DOCX + edited template DOCX (S3 / Azure Blob / disk)

Template metadata

Store “field definitions” in DB:

field_key: Customer.Name

label: “Customer Name”

type: string/date/number

optional formatting rules

Document generation service

Input: template DOCX + JSON data

Output: generated DOCX (and optional PDF)

For merging placeholders, a popular approach in Node is docxtemplater
(If you’re .NET-heavy, you can also do OpenXML-based merging, but Node tooling is often faster to implement.)

Step-by-step implementation
Step 1: Upload DOCX

User uploads a DOCX file

Backend stores it and creates a template_id

DB:

templates(id, name, original_docx_path, working_docx_path, created_by, created_at)

Step 2: Open DOCX in embedded editor

Embed ONLYOFFICE Docs editor in your template page.

What you need:

ONLYOFFICE Document Server running (Docker is common)

Your backend provides:

a file URL that ONLYOFFICE can load

a callback URL that ONLYOFFICE calls when user saves

ONLYOFFICE is designed for this “embed and save back” flow via their Docs API.

Step 3: Add placeholders in “editable mode”

There are two ways:

Approach 3A (fastest): insert placeholder as text

Insert exactly {{FieldName}}

Example: {{Customer.Name}}

Pros: simple
Cons: user can accidentally edit/break the tag

Approach 3B (more robust): use structured fields (content controls)

Some editors allow content controls / protected regions. This is more reliable but more work and depends on editor APIs.

For MVP, do 3A. Most products start there.

Step 4: Capture what placeholders exist

When the user finishes editing:

On save, backend receives the updated DOCX

Backend scans the DOCX for {{...}} tags and stores the unique keys in DB

Now your app can show:

“Detected fields: Customer.Name, Loan.Amount, Loan.StartDate”

Let user map these to your system fields

Step 5: Generate document with real data

When you need to generate:

Load template DOCX

Prepare a JSON object:

{
"Customer": { "Name": "John Doe" },
"Loan": { "Amount": 25000, "StartDate": "2026-02-10" }
}


Run merge → output DOCX (optionally convert to PDF)

If using docxtemplater, it’s built for “replace placeholders in docx using data”.

Important details you should plan for

Placeholder rules: restrict allowed characters (A-Z a-z 0-9 . _) so parsing is reliable.

Escaping: avoid {{ in normal text or allow an escape format.

Repeating tables: eventually you’ll want lists (e.g., line items). Most templating engines support loops, but that’s a “phase 2”.

PDF output: can be done later via LibreOffice headless, ONLYOFFICE conversion, or a paid converter.



Frontend: React / Angular / plain HTML

Editor: ONLYOFFICE Docs embedded

Backend: .NET API (for auth/storage/metadata)

Merge service: Node.js microservice using docxtemplater
(or do it in .NET with OpenXML if you prefer single-stack)