WebAssembly Web API

W3C Proposed Recommendation,

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Daniel Ehrenberg (Igalia)
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This document describes the integration of WebAssembly with the broader web platform.

Part of a collection of related documents: the Core WebAssembly Specification, the WebAssembly JS Interface, and the WebAssembly Web API.

Status of this document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at https://www.w3.org/TR/.

This document was produced by the WebAssembly Working Group as a Proposed Recommendation.

The W3C Membership and other interested parties are invited to review the document and send comments to public-webassembly@w3.org (subscribe, archives) through 27 October 2019. Advisory Committee Representatives should consult their WBS questionnaires

Technical comments are welcomed by the Working Group. While the Candidate Recommendation comment period has ended and the Working Group is unlikley to make substantive modifications to the specification at this stage, future versions of WebAssembly are under active development. GitHub Issues are preferred for discussion of this specification. When filing an issue, please put the text “wasm-web-api” in the title, preferably like this: “[wasm-web-api] …summary of comment…”. All issues and comments are archived.

Publication as a Proposed Recommendation does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

This document was produced by a group operating under the W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

This document is governed by the 1 March 2019 W3C Process Document.

There have been no changes to this document since the publication as a Candidate Recommendation.

According to the Web Platform Tests, which provide an implementation report, this specification has at least two implementations of each feature.

This document builds off of the WebAssembly specification [WEBASSEMBLY] and the WebAssembly JavaScript embedding [WASMJS].

1. Streaming Module Compilation and Instantiation

partial namespace WebAssembly {
  Promise<Module> compileStreaming(Promise<Response> source);
  Promise<WebAssemblyInstantiatedSource> instantiateStreaming(
      Promise<Response> source, optional object importObject);
The compileStreaming(source) method, when invoked, returns the result of compiling a potential WebAssembly response with source.
The instantiateStreaming(source, importObject) method, when invoked, performs the following steps:
  1. Let promiseOfModule be the result of compiling a potential WebAssembly response with source.

  2. Return the result of instantiating the promise of a module promiseOfModule with imports importObject.

To compile a potential WebAssembly response with a promise of a Response source, perform the following steps:

Note: This algorithm accepts a Response object, or a promise for one, and compiles and instantiates the resulting bytes of the response. This compilation can be performed in the background and in a streaming manner. If the Response is not CORS-same-origin, does not represent an ok status, or does not match the `application/wasm` MIME type, the returned promise will be rejected with a TypeError; if compilation or instantiation fails, the returned promise will be rejected with a CompileError or other relevant error type, depending on the cause of failure.

  1. Let returnValue be a new promise

  2. Upon fulfillment of source with value unwrappedSource:

    1. Let response be unwrappedSource’s response.

    2. Let mimeType be the result of extracting a MIME type from response’s header list.

    3. If mimeType is not `application/wasm`, reject returnValue with a TypeError and abort these substeps.

      Note: extra parameters are not allowed, including the empty `application/wasm;`.

    4. If response is not CORS-same-origin, reject returnValue with a TypeError and abort these substeps.

    5. If response’s status is not an ok status, reject returnValue with a TypeError and abort these substeps.

    6. consume response’s body as an ArrayBuffer, and let bodyPromise be the result.

      Note: Although it is specified here that the response is consumed entirely before compilation proceeds, that is purely for ease of specification; implementations are likely to instead perform processing in a streaming fashion. The difference is unobservable, and thus the simpler model is specified.

    7. Upon fulfillment of bodyPromise with value bodyArrayBuffer:

      1. Let stableBytes be a copy of the bytes held by the buffer bodyArrayBuffer.

      2. Asynchronously compile the WebAssembly module stableBytes using the networking task source and resolve returnValue with the result.

    8. Upon rejection of bodyPromise with reason reason:

      1. Reject returnValue with reason.

  3. Upon rejection of source with reason reason:

    1. Reject returnValue with reason.

  4. Return returnValue.

2. Serialization

Web user agents must augment the Module interface with the [Serializable] extended attribute.

The serialization steps, given value, serialized, and forStorage, are:

  1. If forStorage is true, throw a "DataCloneError" DOMException

  2. Set serialized.[[Bytes]] to the sub-serialization of value.[[Bytes]].

  3. Set serialized.[[AgentCluster]] to the current Realm's corresponding agent cluster.

The deserialization steps, given serialized and value, are:

  1. Let bytes be the sub-deserialization of serialized.[[Bytes]].

  2. Set value.[[Bytes]] to bytes.

  3. If targetRealm’s corresponding agent cluster is not serialized.[[AgentCluster]], then throw a "DataCloneError" DOMException.

  4. Compile a WebAssembly module from bytes and set value.[[Module]] to the result.

Engines should attempt to share/reuse internal compiled code when performing a structured serialization, although in corner cases like CPU upgrade or browser update, this might not be possible and full recompilation may be necessary.

Note: The semantics of a structured serialization is as-if the binary source, from which the Module was compiled, is serialized, then deserialized, and recompiled into the target realm. Given the above engine optimizations, structured serialization provides developers explicit control over both compiled-code caching and cross-window/worker code sharing.

3. Developer-Facing Display Conventions

This section is non-normative.

Browsers, JavaScript engines, and offline tools have common ways of referring to JavaScript artifacts and language constructs. For example, locations in JavaScript source code are printed in stack traces or error messages, and are represented naturally as decimal-format lines and columns in text files. Names of functions and variables are taken directly from the sources. Therefore (for example) even though the exact format of implementation-dependent stack trace strings does not always match, the locations are easily understandable and the same across browsers.

To achieve the same goal of a common representations for WebAssembly constructs, the following conventions are adopted.

A WebAssembly location is a reference to a particular instruction in the binary, and may be displayed by a browser or engine in similar contexts as JavaScript source locations. It has the following format:




While the "name" property of an Exported Function instance is specified by the JS API, synthesized function names are also displayed in other contexts like call stacks in debuggers and string representations of stack traces. If a WebAssembly module contains a name section, these names should be used to synthesize a function name as follows:

Note that this document does not specify the full format of strings such as stack frame representations; this allows engines to continue using their existing formats for JavaScript (which existing code may already be depending on) while still printing WebAssembly frames in a format consistent with JavaScript.


Document conventions

Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification.

All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119]

Examples in this specification are introduced with the words “for example” or are set apart from the normative text with class="example", like this:

This is an example of an informative example.

Informative notes begin with the word “Note” and are set apart from the normative text with class="note", like this:

Note, this is an informative note.


Terms defined by this specification

Terms defined by reference


Normative References

ECMAScript® 2018 Language Specification. Current Editor's Draft. URL: https://tc39.github.io/ecma262
Anne van Kesteren. Fetch Standard. Living Standard. URL: https://fetch.spec.whatwg.org/
Anne van Kesteren; et al. HTML Standard. Living Standard. URL: https://html.spec.whatwg.org/multipage/
Domenic Denicola. Writing Promise-Using Specifications. 9 November 2018. TAG Finding. URL: https://www.w3.org/2001/tag/doc/promises-guide
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://tools.ietf.org/html/rfc2119
WebAssembly JS Integration Specification. Draft. URL: https://webassembly.github.io/spec/js-api/
WebAssembly Core Specification. Draft. URL: https://webassembly.github.io/spec/core/
Boris Zbarsky. Web IDL. 15 December 2016. ED. URL: https://heycam.github.io/webidl/

IDL Index

partial namespace WebAssembly {
  Promise<Module> compileStreaming(Promise<Response> source);
  Promise<WebAssemblyInstantiatedSource> instantiateStreaming(
      Promise<Response> source, optional object importObject);