WebAssembly. WebAssembly (Wasm) defines a portable binary-code format and a corresponding text format for executable programs as well as software interfaces for facilitating communication between such programs and their host environment.

The main goal of WebAssembly is to facilitate high-performance applications on web pages, but it is also designed to be usable in non-web environments. It is an open standard intended to support any language on any operating system, and in practice many of the most popular languages already have at least some level of support.

Announced in  and first released in , WebAssembly became a World Wide Web Consortium recommendation on 5 December 2019 and it received the Programming Languages Software Award from ACM SIGPLAN in 2021. The World Wide Web Consortium (W3C) maintains the standard with contributions from Mozilla, Microsoft, Google, Apple, Fastly, Intel, and Red Hat.

History 
The name WebAssembly is intended to seem synonymous with that of the assembly language. The name suggests bringing assembly-like programming to the Web, where it will be executed client-side  by the website-user's computer via the user's web browser. To accomplish this, WebAssembly must be much more hardware-independent than a true assembly language.

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

In March 2017, the design of the minimum viable product (MVP) was declared to be finished and the preview phase ended. 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.

In June 2019, Chrome 75 was released with WebAssembly threads enabled by default.

 WebAssembly 2.0 has been in draft status, which added many SIMD-related instructions and a new v128 datatype, with the ability for functions to return multiple values, and mass memory initialize/copy.

Implementations 
While WebAssembly was initially designed to permit near-native code execution speed in the web browser, it has been considered valuable outside of such, in more generalized contexts. Since WebAssembly's runtime environments (RE) are low-level virtual stack machines (akin to JVM or Flash VM) that may be embedded into host applications, some implementations create standalone runtime environments like  and . WebAssembly runtime environments are embedded in application servers to host "server-side" WebAssembly applications and in other applications to support plug-in-based software extension architectures, e.g., "WebAssembly for Proxies" (proxy-wasm) which specifies a WebAssembly-based ABI for extending proxy servers.

Web browsers 
In November 2017, Mozilla declared support "in all major browsers", after WebAssembly was enabled by default in Edge [Legacy] 16. This support also includes mobile web browsers for iOS and Android.   99% of tracked web browsers  support WebAssembly (version 1.0), more than for its predecessor asm.js.  For some extensions, from the 2.0 draft standard, support may be lower, but still more than 90% of web browsers may already support, e.g. the reference types extension.

Compilers 
WebAssembly implementations usually use either ahead-of-time (AOT) or just-in-time (JIT) compilation, but may also use an interpreter. While the first implementations have landed in web browsers, there are also non-browser implementations for general-purpose use, including Wasmer, Wasmtime or WAMR, wasm3, WAVM, and many others.

Because WebAssembly executables are precompiled, it is possible to use a variety of programming languages to make them. This is achieved either through direct compilation to Wasm, or through an implementation of their corresponding virtual machines in Wasm. Some 40 programming languages are reported to support Wasm as a compilation target.

Emscripten compiles C and C++ to Wasm using Clang as a frontend, replacing LLVM as backend and using  as an optimizer. The Emscripten SDK can compile any LLVM-supported languages (such as C, C++ or Rust, among others) source code into a binary file which runs in the same sandbox as JavaScript code. Emscripten provides bindings for several commonly used environment interfaces like WebGL.

As of version 8, a standalone Clang can compile C and C++ to Wasm. Its initial aim was to support compilation from C and C++, though support for other source languages such as Rust, .NET languages and AssemblyScript (TypeScript-like) is also emerging.

After the MVP release, WebAssembly added support for multithreading and garbage collection (WasmGC, and web browsers including Safari have added support for it), which allowed more efficient compilation for garbage-collecting programming languages like C# (supported via Blazor), F# (supported via Bolero with help of Blazor) and Python.

A number of other languages have some support, including Python, Julia, Ruby and Ring.

A number of systems can compile Java and other JVM languages to JavaScript and WebAssembly. These include CheerpJ, JWebAssembly and TeaVM. Kotlin supports WebAssembly directly.

Limitations 

Web browsers do not permit WebAssembly code to directly manipulate the Document Object Model. Wasm code must defer to JavaScript for this.

In an October 2023 survey of developers, less than half of the 303 participants were satisfied with the state of WebAssembly. A large majority cited the need for improvement in four areas: WASI, debugging support, integration with JavaScript and browser APIs, and build tooling.

For memory-intensive allocations in WebAssembly, there are "grave limitations that make many applications infeasible to be reliably deployed on mobile browsers [..] Currently allocating more than ~300MB of memory is not reliable on Chrome on Android without resorting to Chrome-specific workarounds, nor in Safari on iOS."

All major browsers allow WebAssembly if Content-Security-Policy is not specified, or if "unsafe-eval" is used, but behave differently otherwise. Chrome requires "unsafe-eval", though a worker thread can be a workaround.

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. However, in order to explore these future language extensions, Google Chrome added experimental support for the WebAssembly thread proposal in October 2018.

WebAssembly has been criticized for allowing greater ease of hiding the evidence for malware writers, scammers and phishing attackers; WebAssembly is present on the user's machine only in its compiled form, which "[makes malware] detection difficult". Speed and the easy ability to conceal in WebAssembly have led to its use in hidden crypto mining within the website visitor's device. 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." 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. An April 2021 study from Universität Stuttgart found that since then crypto mining has been marginalized, falling to below 1% of all WebAssembly modules gathered from a wide range of sources, also including the Alexa top 1 million websites. 

As WebAssembly supports only structured control flow, it is amenable toward security verification techniques including symbolic execution.

Performance
Benchmark results vary between implementations and between themselves. Performance was benchmarked early to be around 91% (i.e., 10% slower) for running code, not including load/instantiation time or more recently between 100% and 33% of native rates, and 120% of JavaScript (i.e. 20% faster).  

A 2021 study suggested that WebAssembly, in the versions they tested at that time, was well faster than JavaScript in certain cases and browsers only, such as running a complex function on a small file, e.g. processing a graphics file, but that JavaScript had some optimizations available, e.g. JIT, that WebAssembly did not.

Benchmarking has revealed several other pain-points for WebAssembly, such as poor performance because of no direct access to the DOM, which are being addressed or may be addressed in the future.

WASI 

WebAssembly System Interface (WASI) is a simple interface (ABI and API) designed by Mozilla intended to be portable to any platform. It provides POSIX-like features like file I/O constrained by capability-based security. There are additional proposed ABI/APIs.

WASI is influenced by CloudABI and Capsicum.

, a co-founder of Docker, wrote in 2019, "If WASM+WASI existed in 2008, we wouldn't have needed to create Docker. That's how important it is. WebAssembly on the server is the future of computing."

Specification

Host environment 
The general standard provides core specifications for the JavaScript API and details on embedding.

Virtual machine 
Wasm code (binary code, i.e. bytecode) is intended to be run on a portable virtual stack machine (VM). The VM is designed to be faster to parse and execute than JavaScript and to have compact code representation. Any 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 runs in.

Wasm program 
A Wasm program is designed as a separate module containing collections of various Wasm-defined values and program type definitions. These are provided in either binary or textual format (see below) that have a common structure. Such a module may provide a start function that is executed upon instantiation of a wasm binary.

Instruction set 
The core standard for the binary format of a Wasm program defines an instruction set architecture (ISA) consisting of specific binary encodings of types of operations which are executed by the VM (without specifying how exactly they must be executed). The list of instructions includes standard memory load/store instructions, numeric, parametric, control of flow instruction types and Wasm-specific variable instructions.

The number of opcodes used in the original standard (MVP) was a bit fewer than 200 of the 256 possible opcodes. Subsequent versions of WebAssembly pushed the number of opcodes a bit over 200. The WebAssembly SIMD proposal (for parallel processing) introduces an alternate opcode prefix (0xfd) for 128-bit SIMD. The concatenation of the SIMD prefix, plus an opcode that is valid after the SIMD prefix, forms a SIMD opcode. The SIMD opcodes bring an additional 236 instructions for the "minimum viable product" (MVP) SIMD capability (for a total of around 436 instructions). Those instructions, the "finalized opcodes" are enabled by default across Google's V8 (in Google Chrome), the SpiderMonkey engine in Mozilla Firefox, and the JavaScriptCore engine in Apple's Safari and there are also some additional proposal for instructions for later "post SIMD MVP", and there's also a separate "relaxed-simd" proposal on the table.

These SIMD opcodes are also portable and translate to native instruction sets like x64 and ARM. In contrast, neither Java's JVM nor CIL support SIMD, at their opcode level, i.e. in the standard; both do have some parallel APIs which provide SIMD speedup. There is an extension for Java adding intrinsics for x64 SIMD, that isn't portable, i.e. not usable on ARM or smartphones. Smartphones can support SIMD by calling assembly code with SIMD, and C# has similar support.

Code representation 
In March 2017, the WebAssembly Community Group reached consensus on the initial (MVP) binary format, JavaScript API, and reference interpreter. It defines a WebAssembly binary format (), which is not designed to be used by humans, as well as a human-readable WebAssembly text format () that resembles a cross between S-expressions and traditional assembly languages.

The table below shows an example of a factorial function written in C and its corresponding WebAssembly code after compilation, shown both in  text format (a human-readable textual representation of WebAssembly) and in  binary format (the raw bytecode, expressed below in hexadecimal), that is executed by a Web browser or run-time environment that supports WebAssembly.

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

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. Through , the code above decompiles to:

(module
  (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
      (i64.eqz
        (local.get $p0)) ;; the name $p0 is the same as 0 here
      (then
        (i64.const 1))
      (else
        (i64.mul
          (local.get $p0)
          (call $f0      ;; the name $f0 is the same as 0 here
            (i64.sub
              (local.get $p0)
              (i64.const 1))))))))

A module is implicitly generated by the compiler. The function is referenced by an entry of the type table in the binary, hence a type section and the  emitted by the decompiler. The compiler and decompiler can be accessed online.

See also 
 Architecture Neutral Distribution Format (ANDF)
 UNCOL
 Java bytecode
 Common Language Runtime
 LLVM
 Compilation
 Software portability

Notes

References

External links 

 
 W3C Community Group
 WebAssembly Specification
 WebAssembly Design Documents
  with info on browser compatibility and specifications (WebAssembly JavaScript API)

Assembly languages
Internet properties established in 2015
World Wide Web Consortium standards
Web programming
Bytecodes
Programming languages created in 2015