---
name: malloy-html-data-apps
description: "Build or modify an in-package HTML data app for a Malloy Publisher package (a public/ directory the package serves): design it, write its Publisher.query code, embed it. Hand-authored, no build step."
---
<!--
Copyright (c) Credible Data Inc.
SPDX-License-Identifier: MIT
-->

# In-Package HTML Data Apps

> A package becomes a web app by adding a `public/` directory. Publisher serves those files and gives the page `Publisher.query(...)` to run Malloy against the package's models. No build step, no npm, no framework.

## When this is the right tool

| The user wants | Use |
|---|---|
| A hand-authored HTML/JS dashboard, no toolchain | this skill (an HTML data app) |
| A React app with managed components | the Publisher React SDK (out of scope here) |
| An analyst notebook with charts | a Malloy notebook (`notebooks/<slug>.malloy`) |
| Point-and-click exploration, no code | the Publisher Explorer |

Pick an HTML data app when the user wants full control of the markup and only plain web files.

## Package anatomy

```
my-package/
  publisher.json        # name, version, description
  subscriptions.malloy  # the model(s), stays private
  subscriptions.parquet # data, stays private
  public/               # ONLY this directory is web-served
    index.html
    app.js
    vendor/             # chart library, vendored rather than loaded from a CDN
      chart.umd.js
```

Only `public/` is reachable over the web, at `/environments/<env>/packages/<pkg>/<file>`. Models, data, and `publisher.json` are private and reached only through the query API, which still applies the model's filters, access modifiers, and authorize rules. There is no flag to set: a `public/` directory is what makes a package an app.

## Build sequence

The agent orchestrates these. Each query and chart step hands off to a focused skill.

1. DESIGN IT FIRST. Read `reference/design.md`: who opens it, the decision it serves, the archetype, the form each tile takes, and the depth plan. Skipping this is what produces an app that enumerates the model - a KPI row and a chart grid, whatever the domain - instead of one that answers a question. Hand the brief to the user before scaffolding.
2. READ THE MODEL. Get the model's real source and view names, through your environment's context tool if it has one, or by opening the `.malloy` file directly. Never guess field or view names.
3. SCAFFOLD the package (template below).
4. WRITE THE QUERIES. Read `reference/runtime.md`. Validate each before pasting it into the page, using whatever query tool your environment provides or a running Publisher (see `reference/runtime.md`). Malloy syntax questions go to `skill:malloy-queries`.
5. CHOOSE CHARTS with `skill:malloy-charts` when rendering through `<malloy-render>`; otherwise it is your own chart library drawing the returned rows. Vendor any chart library into `public/` and load it locally, not from a CDN. Two reasons: embedded author JavaScript runs with the viewing user's data authority, and a blocked CDN (agent sandboxes and many corporate networks block them) is easy to miss, because the script never runs and the charts come up empty. The `storefront` example ships its chart library in `public/vendor/` and loads it from `public/index.html` as `./vendor/chart.umd.js`. Copy that, but resolve the path against the page's own directory: a page in a subdirectory (`public/reports/index.html`) needs `../vendor/chart.umd.js`. A wrong relative path 404s and leaves the charts blank, which is the failure you are trying to avoid.
6. EMBED (optional). Read `reference/embedding.md`.
7. PREVIEW with the local authoring loop (below).
8. VERIFY before you call it done (see "What 'done' means" below). This step is not optional.

The scaffold in step 3 only proves the wiring. It is the start, not the deliverable. What you ship is a production app that meets the recipe below.

## What "done" means (production recipe)

A data app you can defend has all of these. Build to this list, not to the scaffold.

- **Real names, never guessed.** Every source, view, and field name comes from the model you read in step 2. A name you derived or assumed is a bug waiting to surface as an empty tile.
- **A real name is not the same as a correct measure.** Using the model's declared measures is right by default, but a wrong one fails far more quietly than a wrong name: an empty tile is obvious, a plausible wrong number is not. Before a measure reaches a headline, confirm it against the raw column values (`reference/design.md` step 2 of the inventory). The case that bites is a predicate like `!= null` over a column whose "absent" value is a sentinel string - measured on a real build, that reported 95.6% of police cases cleared where the true figure was 16.6%, and every check in this skill still passed.
- **DOM-only - never `innerHTML` with interpolated values.** Build every element with `createElement` + `textContent`; do not assign `innerHTML` (or `insertAdjacentHTML`, `document.write`) with any string that contains a model value. Query results render any markup they contain - an XSS vector, and blocked outright under a Trusted-Types CSP. This is a hard build rule, not a lint suggestion: an app that interpolates a model value into `innerHTML` is not done.
- **Modular, not one inline blob.** Split the page into modules per `reference/runtime.md` (pure formatting helpers, a chart layer, your tile/query definitions as data, a thin entry point). One source of truth for each tile's model/source/view, no parallel maps that drift.
- **Every tile handles loading, empty, and error on its own.** One failing query must not blank the page. (`reference/runtime.md`.)
- **Defensible numbers.** Missing ≠ zero (omit the point, don't plot a fake 0); show the latest non-null value for "current"; guard division with `nullif`; convert units explicitly. (`reference/runtime.md`.)
- **Visible assumptions.** When you assume something (two sources joined by month, an in-month proxy that differs from a certified definition) or a metric is incomplete, say so *in the app*: a caption, a footnote, a placeholder card with the reason. The non-technical user cannot see your reasoning; bury a caveat and you have misled them. Don't silently drop a metric you couldn't model. Show a placeholder that names what's missing and why.
- **Built to the design brief, not to the model's shape.** The app matches the archetype, forms, and depth plan from step 1 (`reference/design.md`), carries a real token contract (surfaces, text ramp, semantic colors, a categorical chart palette) that the chart code reads at runtime rather than hardcoding, and puts visible weight on the two or three numbers that lead. A uniform grid of same-size cards, or a tile per view, means the brief was skipped.
- **Vendored libraries.** Chart and helper libraries live in `public/`, loaded locally (step 5).
- **Lazy-load below the fold, once the page has more than about eight query-backed tiles.** ("Many" is not decidable; that is the threshold.) Tiles sitting below the fold is *why* the technique works, not a second trigger for it - almost every real data app scrolls, so reading it as one makes the rule "always" and contradicts the count. Under that threshold, firing every tile at once is cheaper than the machinery. Don't fire every tile's query on load. `reference/lazy-load.md` is the recipe: `IntersectionObserver` (rootMargin ~240px) + a small concurrency cap + reserve each tile's height so lazy tiles don't reflow. Includes the verification trap - on a short/tall-default viewport all tiles intersect at once and you get a false "everything deferred" pass, so test on a deliberately small viewport.

### Verify before you call it done

You are building for someone who cannot tell a correct dashboard from a broken one. Verification is your job, not theirs.

- **Validate every query against the model before wiring it in** (step 4): confirm it compiles and that the column names match what your render code reads.
- **Load the finished page and confirm every tile shows real numbers**: not stuck on "Loading…", not an error, not an empty state you didn't intend. In a headless browser, wait on `load` plus a content selector, not network idle (`publisher.js` holds an SSE stream open; see `reference/runtime.md`). Don't hand-roll this each time - `reference/verification-harness.md` is a copy-adaptable recipe: a mock `sdk/publisher.js` returning canned rows keyed by `(model, query)`, a `python3 -m http.server` webroot, and Playwright assertions (KPIs non-null, no `.is-error`, no stuck `.kit-skeleton`, a chart/table present). It also documents the false-"stuck skeleton" trap (assert after the mock's async delay, never on `networkidle`).
- **Unit-test any non-trivial pure logic** (a month-join, a de-cumulation, a unit conversion). Keep that logic in DOM-free helpers so `node --test` can cover it, and run it.

## Minimal scaffold

`publisher.json` at the package root:

```json
{ "name": "my-package", "version": "0.0.1", "description": "..." }
```

`public/index.html` is a NEW file you create (make the `public/` directory if it does not exist). Load the runtime root-relative, then query. The examples below assume a `subscriptions.malloy` model with a `subscriptions` source; the names are illustrative, so swap in your own model and a view it defines.

Start with the smallest page that proves the wiring, dumping the rows:

```html
<!doctype html>
<title>My dashboard</title>
<pre id="out"></pre>
<script src="/sdk/publisher.js"></script>
<script>
  Publisher.query("subscriptions.malloy", "run: subscriptions -> plan_mix").then((rows) => {
    document.getElementById("out").textContent = JSON.stringify(rows, null, 2);
  });
</script>
```

Then render the rows. This page builds a table from whatever columns the view returns, so it does not depend on the exact field names:

```html
<!doctype html>
<title>Account mix by plan</title>
<table id="t"><thead></thead><tbody></tbody></table>
<script src="/sdk/publisher.js"></script>
<script>
  Publisher.query("subscriptions.malloy", "run: subscriptions -> plan_mix").then((rows) => {
    const t = document.getElementById("t");
    if (!rows.length) { t.textContent = "No rows."; return; }
    const cols = Object.keys(rows[0]);
    const headRow = t.tHead.insertRow();
    for (const c of cols) {
      const th = document.createElement("th");
      th.textContent = c;
      headRow.appendChild(th);
    }
    for (const r of rows) {
      const tr = t.tBodies[0].insertRow();
      for (const c of cols) tr.insertCell().textContent = r[c];
    }
  });
</script>
```

Build row content with `textContent`, not `innerHTML` with model values: an `innerHTML` table renders any markup a value contains. This is the HTML-output side of the don't-trust-interpolated-values rule that `reference/runtime.md` applies to Malloy query strings.

Two invariants break a page most often:

- **The file must live under `public/`.** Publisher serves only `public/`, so a page written anywhere else (for example `/tmp`) is never reachable at `/environments/<env>/packages/<pkg>/<file>`.
- **The script src must be the root-relative `/sdk/publisher.js`**, not a relative path.

A third gotcha: the first argument to `Publisher.query` is the model FILE path (`"subscriptions.malloy"`), not the source name.

## Authoring loop and publishing

Authoring happens locally, then you publish. These are two stages.

### Author locally (with live reload)

Run a local Publisher from the directory that holds your `publisher.config.json` and package folder(s):

```sh
npx @malloy-publisher/server --server_root . --port 4000 --watch-env <env>
```

**`--watch-env` is not optional if you are adding `public/` to a package that is already loaded.** Without it the server COPIES each package into `publisher_data/<env>/<pkg>/` at load time and serves static files from that copy, so a `public/` directory you create afterwards is never seen: every file in it 404s while the rest of the app serves normally. This is the most expensive mistake in this skill, because it looks like your page is broken rather than like the server is stale,.

If you did not start the server (someone handed you a running one), **probe before you build**: write a throwaway file into the package's `public/`, request it, and see whether you get 200 or 404. If it 404s, either restart with `--watch-env <env>` or mirror your files into the served copy after every edit.

`--watch-env <env>` (or `PUBLISHER_WATCH=<env>`) mounts that environment's local-dir packages in place (a symlink, not a copy) and watches them: editing a `.malloy` recompiles the package, and editing a `public/` file live-reloads any open page over an SSE stream. Nothing to wire in the page. The app is served at `http://localhost:4000/environments/<env>/packages/<pkg>/index.html`.

`publisher.config.json` (at `--server_root`) declares the environment, its packages, and its connections:

```json
{
  "frozenConfig": false,
  "environments": [
    {
      "name": "<env>",
      "packages": [{ "name": "<pkg>", "location": "./<pkg>" }],
      "connections": []
    }
  ]
}
```

A local package uses a filesystem `location` (`"./<pkg>"`, relative to the directory holding `publisher.config.json`); a remote one uses a GitHub `tree` URL. If one model in the package fails to compile, the **whole package** fails to load, so a stray notebook/model error blanks every tile. (Common one: a notebook that imports `x.malloy` more than once, such as a `.malloynb` whose cells each `import "x.malloy"`, compiles as one batch, so the repeated import errors `Cannot redefine 'x'`. Import once, at the top.)

### Publishing

Publishing an app is publishing its package: get the package into publishable shape and hand it to your host's publishing workflow. A deployed package serves its `public/` app the same way a local one does, at `/environments/<env>/packages/<pkg>/<file>`. A deployed environment has no `--watch-env` live reload, so the loop there is author, publish, then view.