# The `` tag loads a WebAssembly module the engine needs before the application starts — `Ammo` for physics, `Basis` or `DracoDecoderModule` for compressed assets. :::note[Usage] * It must be a direct child of [``](https://developer.playcanvas.com/user-manual/web-components/tags/pc-app.md). ::: ## Attributes | Attribute | Type | Default | Description | | --- | --- | --- | --- | | `fallback` | String | - | Path to fallback (asm.js) code when WebAssembly is not supported. The Draco decoder ignores it | | `glue` | String | - | Path to the JavaScript glue code for the module | | `name` | String | - | Module name used to reference it in scripts | | `wasm` | String | - | Path to the WebAssembly (.wasm) file | The `name` is case-sensitive — the engine looks the module up by exact name. The modules consumed by the engine are `Ammo` (physics), `Basis` (texture transcoding) and `DracoDecoderModule` (mesh decompression). Getting the case wrong is the one load failure the element cannot report: with `name="ammo"`, the engine throws a `TypeError` as the glue script loads, no `error` event fires, and neither the element nor its `` ever becomes ready. :::note[When these are read] The attributes are read once, when the module starts loading — on the element's first connection to the document, or earlier if a containing `` boots first and collects it. Changing them afterwards has no effect. Creating the element programmatically works as you would expect: set the attributes before appending the element to the document. ::: ## Readiness The element initializes asynchronously and becomes ready once its module has loaded or failed to load: it fires the standard `ready` event and can be awaited with `whenReady('pc-wasm')` — see [Programmatic Access](https://developer.playcanvas.com/user-manual/web-components/programmatic-access.md). (`Basis` is the exception: it is ready as soon as its transcoder starts loading.) Readiness means the load settled, not that it succeeded; a module that fails also fires [`error`](https://developer.playcanvas.com/user-manual/web-components/tags/pc-wasm.md#events). You will rarely need to wait for the element, though: a `` waits for every `` among its children to settle before creating its graphics device, so by the time the app is ready each module has either loaded or reported its failure. A `` nested any deeper still loads, but the app does not wait for it. Readiness is deliberately sticky. A WebAssembly module configures engine-global state that never unloads, so removing the element does not return it to a pending state, and re-inserting it does not load the module again. A `` without a `name` logs a warning and never becomes ready — but it does not block the containing `` from booting. Declare `Ammo` in the markup rather than adding it later. The engine looks for the physics module once, as the application starts, so an app that booted without it runs without physics. ## Events Listen to this event using [`addEventListener()`](https://developer.mozilla.org/en-US/docs/Web/API/EventTarget/addEventListener) or by assigning an event listener to the `oneventname` property of this interface. | Event | Description | | --- | --- | | `error` | An [`ErrorEvent`](https://developer.mozilla.org/en-US/docs/Web/API/ErrorEvent) fired when the module fails to load — from a mistyped URL, say, or a server that refuses it — with the engine's error in `message`. It fires once per element, and the element still becomes ready. A `Basis` module never fires it. | The event does not bubble, so listen on the element itself or with a capture-phase listener on ``. A module that fails to load does not stop the application. The element logs `pc-wasm failed to load module '' - ` to the console, and the containing `` boots without the module — a failed `Ammo` leaves the app running without physics. The engine never tries to load a failed module again, so every `` for that module fails with it: one already waiting when the load fails, and one added later, in a second `` say, which fails straight away. ## Example Loading the `Ammo` physics module. The box only falls because the module is declared — the app waits for it before booting, so physics is ready when the scene starts. Try removing the `` tag and re-running: ```html live-example

``` ## JavaScript Interface You can programmatically create and manipulate `` elements using the [WasmElement API](https://api.playcanvas.com/web-components/classes/WasmElement.html). The element exposes no engine object of its own. Once it is ready the module has been instantiated through the engine's `WasmModule` under its `name` (`Basis` goes through the engine's Basis initializer instead). Ammo's glue script defines the global `Ammo`, so scripts can use it directly; reach other modules through `WasmModule.getInstance(name, callback)`. ## See Also * [``](https://developer.playcanvas.com/user-manual/web-components/tags/pc-rigid-body.md) — physics, the usual reason to load Ammo * [``](https://developer.playcanvas.com/user-manual/web-components/tags/pc-collision.md) — collision shapes for physics * [``](https://developer.playcanvas.com/user-manual/web-components/tags/pc-app.md) — waits for every module before it boots Examples: [Basic Physics](https://playcanvas.github.io/web-components/examples/#basic-physics.html), [Vehicle Physics](https://playcanvas.github.io/web-components/examples/#vehicle-physics.html) and [Video Texture](https://playcanvas.github.io/web-components/examples/#video-texture.html).