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WebAssembly Logo.svg
ParadigmImperative, structured, expression-oriented
Designed byW3C
First appearedMarch 2017; 3 years ago (2017-03)
Typing disciplineStatic
LicenseApache License 2.0
Filename extensions
  • .wat
  • .wasm
Influenced by

WebAssembly (often shortened to Wasm, or just WA) 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, including standalone ones.[5][6][7]

WebAssembly (i.e. WebAssembly Core Specification and WebAssembly JavaScript Interface[8]) became a World Wide Web Consortium recommendation on 5 December 2019[9] and, alongside HTML, CSS, and JavaScript, is the fourth language to run natively in browsers.[10] 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. Blazor is supported via the web assembly Mono-Wasm. There is no direct Document Object Model (DOM) access; however, it is possible to create proxy functions for this, for example through stdweb,[15] web_sys,[16] and js_sys[17] when using the Rust language.

WebAssembly is usually either ahead-of-time (AOT) or just-in-time (JIT) compiled, while there's also available "WebAssembly Micro Runtime (WAMR), an interpreter-based WebAssembly runtime for embedded devices".[18]

The World Wide Web Consortium (W3C) maintains the standard with contributions from Mozilla, Microsoft, Google, Apple, Fastly, Intel, and Red Hat.[19][18]


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]


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.[33][34] Since WebAssembly's runtime environments (RE) are low level virtual stack machines (akin to JVM or Flash VM) that can be embedded into host applications, some of them have found a way to standalone runtime environments like Wasmtime and Wasmer.[35]

Web browsers[edit]

In November 2017, Mozilla declared support "in all major browsers"[36] (by now all major on mobile and desktop), after WebAssembly was enabled by default in Edge 16.[37] The support includes mobile web browsers for iOS and Android. As of October 2020, 92.06% of installed browsers (92.93% of desktop browsers and 93.44% of mobile browsers) support WebAssembly.[38] But for older browsers, Wasm can be compiled into asm.js by a JavaScript polyfill.[39]


Because WebAssembly executables are precompiled, it is possible to use a variety of programming languages to make them.[40] 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.[41]

Emscripten compiles C and C++ to Wasm[28] using the Binaryen and LLVM as backend.[42]

As of version 8 a standalone Clang can compile C and C++ to Wasm.[43]

Its initial aim is to support compilation from C and C++,[44] though support for other source languages such as Rust, .NET languages[45][46][41] and AssemblyScript[47] (TypeScript-like) is also emerging. After the MVP release, there are plans to support multithreading and garbage collection[48][49] which would make WebAssembly a compilation target for garbage-collected programming languages like C# (supported via Blazor), F# (supported via Bolero[50] with help of Blazor), Python, and even JavaScript where the browser's just-in-time compilation speed is considered too slow. A number of other languages have some support including Python,[51] Java,[52] Julia,[53][54][55] and Ruby,[56] as well as Go.

Security considerations[edit]

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.[57][58][59] However, in order to explore these future language extensions, Google Chrome added experimental support for the WebAssembly thread proposal in October 2018.[60]

WebAssembly has been criticized for allowing greater ease of hiding the evidence for malware writers, scammers and phishing attackers; WebAssembly is only present on the user's machine in its compiled form, which "[makes malware] detection difficult".[61] The speed and concealability of WebAssembly have led to its use in hidden crypto mining on the website visitor's device.[61][62][57] 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."[57] 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.[63][64]

The ability to effectively obfuscate large amounts of code can also be used to disable ad blocking and privacy tools that prevent web tracking like Privacy Badger.

As WebAssembly only supports structured control flow, it is amenable toward security verification techniques including symbolic execution. Current efforts in this direction include the Manticore symbolic execution engine.[65]


WebAssembly System Interface (WASI) is a simple interface (ABI and API) designed by Mozilla intended to be portable to any platform.[66] It provides POSIX-like features like file I/O constrained by capability-based security.[67][68] There are also a few other proposed ABI/APIs.[69][70]

WASI is influenced by CloudABI and Capsicum.


Host environment[edit]

The general standard provides core specifications for JavaScript API and details on embedding.[3]

Virtual machine[edit]

Wasm code (binary or bytecode) is intended to be run on a portable virtual stack machine (VM).[71] The VM is designed to be faster to parse and execute than JavaScript and to have a compact code representation.[44] An external functionality (like syscalls) that may be expected by Wasm binary code is not stipulated by the standard. It rather provides a way to deliver interfacing via modules by the host environment that the VM implementation runs in.[72][35]

Wasm program[edit]

A Wasm program is designed to be a separate module containing collections of various Wasm-defined values and program type definitions. These are expressed in either binary or textual format (see below) that both have a common structure.[73]

Instruction set[edit]

The core standard for the binary format of a Wasm program defines an instruction set architecture consisting of specific binary encodings of types of operations which are executed by the VM. It doesn't specify how exactly they must be executed by the VM however.[74] The list of instructions includes standard memory load/store instructions, numeric, parametric, control of flow instruction types and Wasm-specific variable instructions.[75]

Code representation[edit]

In March 2017, the WebAssembly Community Group reached consensus on the initial (MVP) binary format, JavaScript API, and reference interpreter.[76] It defines a WebAssembly binary format (.wasm), which is not designed to be used by humans, as well as a human-readable WebAssembly text format (.wat) that resembles a cross between S-expressions and 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:[77]

The same source code in C, assembly, and Wasm
C input source Linear assembly bytecode (intermediate representation) Wasm binary encoding (hexadecimal bytes)
int factorial(int n) {
  if (n == 0)
    return 1;
    return n * factorial(n-1);
; magic number
; type for (func (param i64) (result i64))
; function section
; code section start
(func (param i64) (result i64)
  local.get 0
  if (result i64)
      i64.const 1
      local.get 0
      local.get 0
      i64.const 1
      call 0
; module end, size fixups
00 61 73 6D 01 00 00 00
01 00 01 60 01 73 01 73 06
03 00 01 00 02
0A 00 01
00 00
20 00
04 7E
42 01
20 00
20 00
42 01
10 00
0B 15 17

All integer constants are encoded using a space-efficient, variable-length LEB128 encoding.[78]

The WebAssembly text format is more canonically written in a folded format using s-expressions. For instructions and expressions, this format is purely syntactic sugar and has no behavioral differences with the linear format.[79] Through wasm2wat, the code above decompiles to:

  (type $t0 (func (param i64) (result i64)))
  (func $f0 (type $t0) (param $p0 i64) (result i64)
    (if $I0 (result i64) ; $I0 is an unused label name
        (local.get $p0)) ; the name $p0 is the same as 0 here
        (i64.const 1))
          (local.get $p0)
          (call $f0      ; the name $f0 is the same as 0 here
              (local.get $p0)
              (i64.const 1))))))))

Note that a module is implicitly generated by the compiler. The function is actually referenced by an entry of the type table in the binary, hence a type section and the type emitted by the decompiler.[80] The compiler and decompiler can be accessed online.[81]


  • 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.
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  1. ^ According to official documentation, the Emscripten SDK may be used to create .wasm files which then may be executed in a web browser.[11][12][13] Even though Emscripten can consume various languages when using Clang, some problems may arise.[14]


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