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WebAssembly (often shortened to Wasm) is an open standard that defines a portable binary code format for executable programs, and a corresponding textual assembly language, as well as interfaces for facilitating interactions between such programs and their host environment.[2][3][4][5] The main goal of WebAssembly is to enable high performance applications on web pages, but the format is designed to be executed and integrated in other environments as well.[6][7]

Wasm does not replace JavaScript; in order to use Wasm in browsers, users may use Emscripten SDK to compile C++ (or any other LLVM-supported language such as D or Rust) source code into a binary file which runs in the same sandbox as regular JavaScript code;[1] Emscripten provides bindings for several commonly used environment interfaces like WebGL; it has access only to an expandable memory and a small number of scalar values. There is no direct Document Object Model (DOM) access; however, it is possible to create proxy functions for this, for example through stdweb,[12] web_sys,[13] and js_sys.[14]

The World Wide Web Consortium (W3C) maintains the standard with contributions from Mozilla, Microsoft, Google, and Apple.[15]

ParadigmImperative, structured, expression-oriented
Designed byW3C
  • W3C
  • Mozilla
  • Microsoft
  • Google
  • Apple
First appearedMarch 2017 (2017-03)
Typing disciplineStatic
LicenseApache License 2.0
Filename extensions
  • .wat
  • .wasm
Websitewebassembly.org [70]
Influenced by
  • asm.js
  • PNaCl


WebAssembly was first announced in 2015,[16] and the first demonstration was executing Unity's Angry Bots in Firefox,[17] Google Chrome,[18] and Microsoft Edge.[19] The precursor technologies were asm.js from Mozilla and Google Native Client,[20][21] and the initial implementation was based on the feature set of asm.js.[22]

In March 2017, the design of the minimum viable product (MVP) was declared to be finished and the preview phase ended.[23] In late September 2017, Safari 11 was released with support. In February 2018, the WebAssembly Working Group published three public working drafts for the Core Specification, JavaScript Interface, and Web API.[24][25][26][27]


In November 2017, Mozilla declared support "in all major browsers"[28] (by now all major on mobile and desktop), after WebAssembly was enabled by default in Edge 16.[29] The support includes mobile web browsers for iOS and Android. As of September 2019, 87.42% of installed browsers (89.39% of desktop browsers and 87.4% of mobile browser) support WebAssembly.[30] But for older browsers, Wasm can be compiled into asm.js by a JavaScript polyfill.[31]

Because WebAssembly executables are precompiled, it is possible to use a variety of programming languages to make them.[32] This is achieved either through direct compilation to Wasm, or through implementation of the corresponding virtual machines in Wasm. There have been around 40 programming languages reported to support Wasm as a compilation target.[33]

Emscripten can compile C and C++ to Wasm[23] using LLVM in the backend.

Its initial aim is to support compilation from C and C++,[34] though support for other source languages such as Rust and .NET languages is also emerging.[35][36][33] After the MVP release, there are plans to support multithreading and garbage collection[37][38] which would make WebAssembly a compilation target for garbage-collected programming languages like C# (supported via Blazor) and F# (supported via Bolero[39] with help of Blazor); Java, Julia,[40][41][42] Ruby,[43] as well as Go.

Security considerations

In June 2018, a security researcher presented the possibility of using WebAssembly to circumvent browser mitigations for Spectre and Meltdown security vulnerabilities once support for threads with shared memory is added. Due to this concern, WebAssembly developers put the feature on hold.[44][45][46] Thread support was eventually added in October 2018.[47]


The general standards provide core specifications for JavaScript and Web embedding.[4]

While WebAssembly was initially designed to enable near-native code execution speed in the web browser, it has been considered valuable outside of such, in more generalized contexts.[48][49]

WebAssembly System Interface (WASI) is an ABI designed by Mozilla intended to define a simpler ABI for WebAssembly that can be used in any platform.[50] There are also a few other proposed ABI APIs.[51][52]


Stack machine

Wasm code is intended to be run on a portable abstract structured virtual stack machine (VM).[53] The VM is designed to be faster to parse than JavaScript, as well as faster to execute and to enable very compact code representation.[34]

Instruction set

The core standard defines a unique Instruction Set Architecture consisting of specific binary encoding and which is intended to be executed by VM. However it doesn't specify how exactly they must be invoked by it.[54]


In March 2017, the WebAssembly Community Group reached consensus on the initial (MVP) binary format, JavaScript API, and reference interpreter.[55] It defines a WebAssembly binary format, which is not designed to be used by humans, as well as a human-readable linear assembly bytecode format that resembles traditional assembly languages.

The table below represents three different views of the same source code input from the left, as it is converted to a Wasm intermediate representation, then to Wasm binary instructions:[56]

C input sourceLinear assembly bytecode
(intermediate representation)
Wasm binary encoding
(hexadecimal bytes)
get_local 0
if (result i64)
i64.const 1
get_local 0
get_local 0
i64.const 1
call 0
20 00
04 7E
42 01
20 00
20 00
42 01
10 00

The WebAssembly text format can also be written in a folded format using s-expressions. This format is purely syntactic sugar and has no behavioral differences with the linear format.[57] An example is shown below:


  • Haas, Andreas; Rossberg, Andreas; Schuff, Derek L.; Titzer, Ben L.; Gohman, Dan; Wagner, Luke; Zakai, Alon; Bastien, JF; Holman, Michael (June 2017). "Bringing the web up to speed with WebAssembly" [71] . Proceedings of the 38th ACM SIGPLAN Conference on Programming Language Design and Implementation. Association for Computing Machinery: 185–200. doi:10.1145/3062341.3062363 [72] .

  • Watt, Conrad (2018). "Mechanising and Verifying the WebAssembly Specification" [73] (PDF). ACM SIGPLAN International Conference on Certified Programs and Proofs. ACM. 7. doi:10.1145/3167082 [74] .


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Citation Linkwebassembly.github.io"Introduction — WebAssembly 1.0". webassembly.github.io. Retrieved 18 June 2019. ... this specification is complemented by additional documents defining interfaces to specific embedding environments such as the Web. These will each define a WebAssembly application programming interface (API) suitable for a given environment.
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Citation Linkportal.issn.orgHaas, Andreas; Rossberg, Andreas; Schuff, Derek L.; Titzer, Ben L.; Holman, Michael; Gohman, Dan; Wagner, Luke; Zakai, Alon; Bastien, JF (14 June 2017). "Bringing the Web Up to Speed with WebAssembly". SIGPLAN Not. 52 (6): 185–200. doi:10.1145/3140587.3062363. ISSN 0362-1340. While the Web is the primary motivation for WebAssembly, nothing in its design depends on the Web or a JavaScript environment. It is an open standard specifically designed for embedding in multiple contexts, and we expect that stand-alone implementations will become available in the future.
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