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.[1][2][3][4] 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.[5][6]

ParadigmImperative, structured, expression-oriented
Designed byW3C
First appearedMarch 2017 (2017-03)
Typing disciplineStatic
LicenseApache License 2.0
Filename extensions
  • .wat
  • .wasm
Influenced by

WebAssembly became a World Wide Web Consortium recommendation on December 05, 2019[7] and, alongside HTML, CSS, and JavaScript, it is the fourth language to run natively in browsers[8]. 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;[note 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,[13] web_sys,[14] and js_sys[15] when using Rust language.

The World Wide Web Consortium (W3C) maintains the standard with contributions from Mozilla, Microsoft, Google, and Apple.[16]

A June 2019 study from the Technische Universität Braunschweig, analyzed the usage of WebAssembly in the Alexa top 1 million websites and found the most prevalent use was for malicious crypto mining.[17][18][19]


WebAssembly was first announced in 2015,[20] and the first demonstration was executing Unity's Angry Bots in Firefox,[21] Google Chrome,[22] and Microsoft Edge.[23] The precursor technologies were asm.js from Mozilla and Google Native Client,[24][25] and the initial implementation was based on the feature set of asm.js.[26] The asm.js technology already provides near-native code execution speeds[27] and can be considered a viable alternative for browsers that don't support WebAssembly or have it disabled for security reasons.

In March 2017, the design of the minimum viable product (MVP) was declared to be finished and the preview phase ended.[28] 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.[29][30][31][32]


In November 2017, Mozilla declared support "in all major browsers"[33] (by now all major on mobile and desktop), after WebAssembly was enabled by default in Edge 16.[34] The support includes mobile web browsers for iOS and Android. As of December 2019, 88.66% of installed browsers (89.93% of desktop browsers and 89.5% of mobile browsers) support WebAssembly.[35] But for older browsers, Wasm can be compiled into asm.js by a JavaScript polyfill.[36]

Because WebAssembly executables are precompiled, it is possible to use a variety of programming languages to make them.[37] 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.[38]

Emscripten compiles C and C++ to Wasm[28] using the LLVM backend.[39]

As of version 8[40] Clang alone can compile C and C++ to Wasm.

Its initial aim is to support compilation from C and C++,[41] though support for other source languages such as Rust and .NET languages is also emerging.[42][43][38] After the MVP release, there are plans to support multithreading and garbage collection[44][45] which would make WebAssembly a compilation target for garbage-collected programming languages like C# (supported via Blazor) and F# (supported via Bolero[46] with help of Blazor); a number of other languages have some support including Java, Julia,[47][48][49] Ruby,[50] 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.[51][52][53] Thread support was eventually added in October 2018.[54]

WebAssembly has been criticized for allowing greater ease of hiding the evidence for malware writers, scammers and phishing attackers. In addition, WebAssembly is only present on the user's machine in its compiled form, which "[makes malware] detection difficult".[55] The speed and concealability of WebAssembly have led to its use in hidden crypto mining on the website visitor's device.[55][56][51] Coinhive, a now defunct service facilitating cryptocurrency mining in website visitors' browsers, claims their "miner uses WebAssembly and runs with about 65% of the performance of a native Miner."[51] A June 2019 study from the Technische Universität Braunschweig, analyzed the usage of WebAssembly in the Alexa top 1 million websites and found the prevalent use was for malicious crypto mining, and that malware accounted for more than half of the WebAssembly-using websites studied.[57][58]


The general standards provide core specifications for JavaScript and Web embedding.[3]

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.[59][60]

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.[61] There are also a few other proposed ABI APIs.[62][63]


Virtual machine

Wasm code (binary or bytecode) is intended to be run on a portable virtual stack machine (VM).[64] The VM is designed to be faster to parse and execute than JavaScript and to have a compact code representation.[41]

Wasm program

A Wasm program is designed to be a separate module containing collection of various wasm-defined value and program types definitions expressed either in binary or textual format (see below) that both have a common structure.[65]

Instruction set

The core standard for binary format of wasm program defines Instruction Set Architecture consisting of specific binary encoding of types of operations which are executed by the VM. It doesn't specify how exactly they must be executed by the VM however.[66] The list of instructions includes standard memory load/store instructions, numeric, parametric, control of flow instruction types and wasm-specific variable instructions.[67]

Code representation

In March 2017, the WebAssembly Community Group reached consensus on the initial (MVP) binary format, JavaScript API, and reference interpreter.[68] 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:[69]

C input sourceLinear assembly bytecode
(intermediate representation)
Wasm binary encoding
(hexadecimal bytes)
int factorial(int n) {
  if (n == 0)
    return 1;
    return n * factorial(n-1);
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.[70] An example is shown below:

  (import "math" "exp" (func $exp (param f64) (result f64)))
  (func (export "doubleExp") (param $0 f64) (result f64)
      (call $exp
        (get_local $0)
      (f64.const 2)


  • 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". Proceedings of the 38th ACM SIGPLAN Conference on Programming Language Design and Implementation. Association for Computing Machinery: 185–200. doi:10.1145/3062341.3062363. ISBN 9781450349888.
  • Watt, Conrad (2018). "Mechanising and Verifying the WebAssembly Specification" (PDF). ACM SIGPLAN International Conference on Certified Programs and Proofs. ACM. 7: 53–65. doi:10.1145/3167082. ISBN 9781450355865.


  1. According to official documentation the Emscripten SDK may be used to create .wasm files which then may be executed in web browser.[9][10][11] Even though Emscripten can consume various languages when using Clang some problems may arise.[12]


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  4. "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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