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Web Embedding

Unsurprisingly, one of WebAssembly's primary purposes is to run on the Web, for example embedded in Web browsers (though this is not its only purpose).

This means integrating with the Web ecosystem, leveraging Web APIs, supporting the Web's security model, preserving the Web's portability, and designing in room for evolutionary development. Many of these goals are clearly reflected in WebAssembly's high-level goals. In particular, WebAssembly MVP will be no looser from a security point of view than if the module was JavaScript.

More concretely, the following is a list of points of contact between WebAssembly and the rest of the Web platform that have been considered:

JavaScript API

A JavaScript API is provided which allows JavaScript to compile WebAssembly modules, perform limited reflection on compiled modules, store and retrieve compiled modules from offline storage, instantiate compiled modules with JavaScript imports, call the exported functions of instantiated modules, alias the exported memory of instantiated modules, etc.

The Web embedding includes additional methods useful in that context. In non-web embeddings, these APIs may not be present.

Additional Web Embedding API

This section historically contained the description of WebAssembly's Web API.

For the current description, see the normative documentation.

Modules

WebAssembly's modules allow for natural integration with the ES6 module system.

Names

A WebAssembly module can have imports and exports, which are identified using UTF-8 byte sequences. The most natural Web representation of a mapping of export names to exports is a JS object in which each export is a property with a name encoded in UTF-16. A WebAssembly module fails validation on the Web if it has imports or exports whose names do not transcode cleanly to UTF-16 according to the following conversion algorithm, assuming that the WebAssembly name is in a Uint8Array called array:

function convertToJSString(array)
{
  var string = "";
  for (var i = 0; i < array.length; ++i)
    string += String.fromCharCode(array[i]);
  return decodeURIComponent(escape(string));
}

This performs the UTF8 decoding (decodeURIComponent(escape(string))) using a common JS idiom. Transcoding failure is detected by decodeURIComponent, which may throw URIError. If it does, the WebAssembly module will not validate. This validation rule is only mandatory for Web embedding.

Security

WebAssembly's security model should depend on the same-origin policy, with cross-origin resource sharing (CORS) and subresource integrity to enable distribution through content distribution networks and to implement dynamic linking.

SIMD

Once SIMD is supported WebAssembly would:

  • Be statically typed analogous to SIMD.js-in-asm.js;
  • Reuse specification of operation semantics (with TC39);
  • Reuse backend implementation (same IR nodes).

GC

Once GC is supported, WebAssembly code would be able to reference and access JavaScript, DOM, and general WebIDL-defined objects.