---
title: Variables
description: "Parameterize compositions so the same source can render different content."
---

Variables let you declare named, typed slots in a composition and fill them at render time — from a parent composition, from the CLI, or from an API call. A card composition that takes `title` and `color` can be embedded a hundred times with a hundred different values without duplicating any HTML.

## Declaring Variables

Add `data-composition-variables` to the `<html>` root of any composition. Its value is a JSON array of variable declarations — one object per variable:

```html compositions/card.html
<html data-composition-variables='[
  {"id":"title",  "type":"string",  "label":"Title",  "default":"Hello"},
  {"id":"color",  "type":"color",   "label":"Color",  "default":"#111827"},
  {"id":"price",  "type":"number",  "label":"Price",  "default":0,        "unit":"$"},
  {"id":"featured","type":"boolean","label":"Featured","default":false},
  {"id":"plan",   "type":"enum",    "label":"Plan",   "default":"pro",
   "options":[{"value":"pro","label":"Pro"},{"value":"enterprise","label":"Enterprise"}]}
]'>
```

Every declaration requires four fields: `id`, `type`, `label`, and `default`. `id` must be unique within the composition.

## Variable Types

| Type | `default` value | Extra options |
|------|----------------|---------------|
| `string` | `"some text"` | `placeholder?: string`, `maxLength?: number` |
| `number` | `0` | `min?: number`, `max?: number`, `step?: number`, `unit?: string` |
| `color` | `"#rrggbb"` | — |
| `boolean` | `true` / `false` | — |
| `enum` | one of the option values | `options: [{value: string, label: string}]` |

The Studio editing UI uses `label`, `type`, and the type-specific options to render the right input widget for each variable.

## What can be a variable

Variables come in two layers. The five [declared types](#variable-types) above cover typed primitive data — strings, numbers, colors, booleans, enums. For everything else, a `string` variable holding a URL is the escape hatch: your composition reads the URL and assigns it to whatever DOM element needs it.

### Parameterizing media assets

The same composition can render different images, video clips, or audio tracks just by swapping URLs through a string variable:

```html compositions/product-card.html
<html data-composition-variables='[
  {"id":"productImage","type":"string","label":"Product image URL","default":"https://cdn.example.com/products/default.png"},
  {"id":"productName","type":"string","label":"Product name","default":"Untitled"}
]'>
  <body>
    <div data-composition-id="product-card" data-width="1920" data-height="1080" data-duration="5">
      <img class="product-img" alt="" />
      <h1 class="product-name"></h1>

      <script>
        const {
          productImage = "https://cdn.example.com/products/default.png",
          productName = "Untitled",
        } = __hyperframes.getVariables();
        const root = document.querySelector('[data-composition-id="product-card"]');
        root.querySelector(".product-img").src = productImage;
        root.querySelector(".product-name").textContent = productName;
      </script>
    </div>
  </body>
</html>
```

<Note>
The runtime probes the DOM after your composition script runs, so a `<video>` or `<audio>` `src` assigned at runtime from a variable is discovered and pre-extracted for the render. No extra wiring required — just set the `src` from your variable.
</Note>

The same pattern covers the three media element types:

- **`<img src>`** — assign from a string variable. Chrome fetches it during capture like any other image; no extra config.
- **`<video src>`** — assign from a string variable, but keep the timing attributes (`data-start`, `data-duration`, `data-track-index`, `data-has-audio`) on the element itself. The probe phase scans `video[data-start]` elements after your script runs and reads the resolved `src` for pre-extraction.
- **`<audio src>`** — same as video. The audio is decoded during capture and mixed into the final output.

Pass assets as URL references your composition resolves at render time; don't inline base64. URL-shaped assets travel cleanly through both the local renderer and the Lambda surface — see [Templates on Lambda](/deploy/templates-on-lambda#working-with-large-variables) for the 256 KiB execution-input cap on distributed renders.

### Swapping media: do you need to vary duration too?

A common follow-up: if a variable swaps a `<video>` to a different clip, does `data-duration` need to change too? Usually no. `data-duration` is optional on `<video>` and `<audio>` — leave it off and the renderer ffprobes the source and uses its natural length:

```html compositions/hero.html
<video id="hero" data-start="0" data-track-index="0"></video>
<script>
  document.getElementById("hero").src = __hyperframes.getVariables().heroVideo;
</script>
```

If you need to clamp or pin the clip to a specific length per render — for example, to keep downstream timing stable across clips of different source lengths — expose duration as its own `number` variable and apply it via the same script:

```html compositions/hero.html
<video id="hero" data-start="0" data-track-index="0"></video>
<script>
  const { heroVideo, heroDuration } = __hyperframes.getVariables();
  const el = document.getElementById("hero");
  el.src = heroVideo;
  if (heroDuration !== undefined) {
    el.setAttribute("data-duration", String(heroDuration));
  }
</script>
```

The probe phase reads `data-duration` from the live DOM after your script runs, so an attribute written programmatically behaves identically to one baked into the source HTML.

## What can't be a variable

A small set of inputs are read once from the source HTML or from the CLI / SDK, with no live-DOM re-read — no script (and therefore no variable) can change them:

| What | Mechanism (not a variable) |
|------|----------------------------|
| Composition dimensions | `data-width` / `data-height` on the composition element — parsed from the source HTML at compile time, not from the live DOM |
| Frame rate | `--fps` flag on `hyperframes render`, or `config.fps` in the SDK |
| Output format / codec / quality | `--format` / `--codec` / `--quality` flags, or the SDK equivalents |
| A sibling or parent composition's variables | Variables are per-composition; use [`data-variable-values`](#per-instance-overrides-sub-compositions) on each sub-comp host element to pass overrides |

The deeper rule: variables are runtime values your script applies to the DOM. They can drive anything the renderer reads from the live DOM after that script runs — text, colors, media `src`, even clip `data-duration` as shown above. They can't change inputs the renderer reads once at compile time (dimensions) or that live entirely outside the composition (CLI flags, encoder settings).

## Reading Variables at Runtime

Inside any composition script, call `window.__hyperframes.getVariables()` to get the resolved variable values. The return type is `Partial<Record<string, unknown>>` — use destructuring with defaults matching the declared `default` values:

```html compositions/card.html
<html data-composition-variables='[
  {"id":"title","type":"string","label":"Title","default":"Untitled"},
  {"id":"color","type":"color","label":"Color","default":"#111827"}
]'>
  <body>
    <div data-composition-id="card" data-width="1920" data-height="1080">
      <h1 class="card-title"></h1>

      <style>
        [data-composition-id="card"] { --card-color: #111827; }
        [data-composition-id="card"] .card-title { color: var(--card-color); }
      </style>

      <script>
        const { title = "Untitled", color = "#111827" } = __hyperframes.getVariables();
        const root = document.querySelector('[data-composition-id="card"]');
        root.querySelector(".card-title").textContent = title;
        root.style.setProperty("--card-color", color);
      </script>
    </div>
  </body>
</html>
```

`__hyperframes.getVariables()` is a shorthand for `window.__hyperframes.getVariables()` and works in both top-level and sub-composition scripts. The runtime automatically scopes sub-compositions so each instance sees its own resolved values.

## Per-instance Overrides (Sub-compositions)

When embedding a composition inside another, use `data-variable-values` on the host element to pass a JSON object of override values for that particular instance:

```html index.html
<div
  data-composition-id="card-pro"
  data-composition-src="compositions/card.html"
  data-start="0"
  data-track-index="1"
  data-variable-values='{"title":"Pro","color":"#ff4d4f"}'
></div>
<div
  data-composition-id="card-enterprise"
  data-composition-src="compositions/card.html"
  data-start="card-pro"
  data-track-index="1"
  data-variable-values='{"title":"Enterprise","color":"#22c55e"}'
></div>
```

Both host elements point to the same `card.html` source, but each instance receives different values. The runtime merges the host's `data-variable-values` over the sub-comp's declared defaults on a per-instance basis — the same sub-composition can run with completely different content simultaneously.

## CLI Overrides (Top-level Renders)

Pass variable values at render time with `--variables` or `--variables-file`. These override the declared defaults for the top-level composition:

```bash Terminal
# Inline JSON
npx hyperframes render --variables '{"title":"Q4 Report","color":"#1d4ed8"}' --output q4.mp4

# JSON file
npx hyperframes render --variables-file ./vars.json --output out.mp4

# Fail on undeclared or mistyped variables
npx hyperframes render --variables '{"title":"Q4 Report"}' --strict-variables --output out.mp4
```

`--strict-variables` turns variable warnings into errors. Any variable in `--variables` that is not declared in `data-composition-variables`, or whose value does not match the declared type, causes the render to exit non-zero. Useful in CI pipelines where an undeclared variable key likely indicates a typo or a schema mismatch.

<Note>
  CLI overrides apply only to the top-level composition. Sub-composition variables are controlled by `data-variable-values` on each host element.
</Note>

## Layering and Precedence

Variable values are resolved by merging three sources, lowest to highest precedence:

| Source | Precedence | Where declared |
|--------|-----------|---------------|
| Declared defaults | Lowest | `data-composition-variables` on `<html>` |
| Per-instance host overrides | Middle | `data-variable-values` on the sub-comp host element |
| CLI `--variables` flag | Highest | `hyperframes render --variables '{...}'` |

A missing key at any layer falls through to the next lower layer. If no layer provides a value, the declared `default` is used.

## Validation

The linter checks variable declarations statically:

```bash Terminal
npx hyperframes lint
```

It catches malformed JSON, missing required fields (`id`, `type`, `label`, `default`), and type mismatches between `type` and the `default` value. Fix lint errors before rendering — they indicate the runtime will be unable to resolve variables correctly.

At render time, the CLI validates `--variables` against the schema and reports issues as warnings (or errors with `--strict-variables`):

- **undeclared** — a key in `--variables` has no matching `id` in `data-composition-variables`
- **type-mismatch** — the value's JavaScript type does not match the declared `type` (e.g. a string where a number is expected)
- **enum-out-of-range** — an enum value is not in the declared `options` list

## Inspecting Variables Programmatically

If you are building tooling on top of `@hyperframes/core`, the variable declarations are readable without rendering:

```typescript
import { extractCompositionMetadata } from "@hyperframes/core";
import { readFileSync } from "node:fs";

const html = readFileSync("compositions/card.html", "utf8");
const { variables } = extractCompositionMetadata(html);
// variables is CompositionVariable[]
```

This is the same API the Studio editing UI uses to build the variables panel for each composition.

## Next Steps

<CardGroup cols={2}>
  <Card title="Data Attributes" icon="code" href="/concepts/data-attributes">
    Full reference for data-composition-variables and data-variable-values attributes
  </Card>
  <Card title="Compositions" icon="layer-group" href="/concepts/compositions">
    How nested compositions use variables for reuse
  </Card>
  <Card title="Rendering" icon="play" href="/guides/rendering">
    CLI flags for passing variables at render time
  </Card>
  <Card title="CLI Reference" icon="terminal" href="/packages/cli">
    All CLI commands and flags
  </Card>
</CardGroup>
