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asm.js is a restricted subset of JavaScript designed to make compiled code easier for JavaScript engines to analyze and optimize. It remained ordinary JavaScript, not a separate binary format. Today, MDN considers the asm.js specification deprecated and points developers toward WebAssembly for high-performance browser code.
What is asm.js?
MDN defines asm.js as “a specification defining a subset of JavaScript that is highly optimizable.” (MDN: asm.js – Game development) The subset uses deliberately predictable patterns, including simple control flow, typed integer and floating-point operations, function calls, and heap access. It avoids many dynamic JavaScript features that can require general object allocation.
That restriction made asm.js useful as a compiler target. A compiler could emit code in a form engines could more readily analyze, and an engine that recognized the patterns could apply specialized optimizations. The output was still JavaScript, so it could run in a JavaScript engine without a separate asm.js runtime.
Why did developers use asm.js?
asm.js helped bring code written in languages such as C and C++ to the browser. Emscripten, an LLVM-to-JavaScript compiler, could compile LLVM bytecode into asm.js for browser execution. (MDN: Game development tools) This gave developers a route for porting native code to the web while relying on JavaScript engines to execute the result.
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The goal was efficient execution, but no universal speed advantage follows from using asm.js. Actual performance depended on the browser engine, hardware, and workload; MDN describes the goal qualitatively rather than providing a guaranteed speedup.
How is asm.js different from WebAssembly?
Both have been used to run compiled code in web applications, but they are different output formats with different integration models.
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| Aspect | asm.js | WebAssembly |
|---|---|---|
| Representation | A restricted subset of JavaScript that remains JavaScript. (MDN: asm.js – Game development) | A separate, assembly-like bytecode format for compiled languages. (MDN: WebAssembly) |
| Compiler output and integration | JavaScript source that a JavaScript engine can run; specialized optimization depends on the engine. | A Wasm module that JavaScript APIs load and expose to a web application. Emscripten can produce a module plus JavaScript glue code; in MDN’s described model, Wasm calls JavaScript for web APIs such as DOM access. (MDN: WebAssembly concepts) |
| Current guidance | MDN labels the specification deprecated. (MDN: asm.js – Game development) | MDN points developers toward WebAssembly as an alternative for high-performance browser code. (MDN: asm.js – Game development) |
| Performance | No guaranteed advantage; results depend on implementation, hardware, and workload. | No current benchmark comparison is established here; evaluate the target browser and workload rather than assuming a fixed ranking. |
WebAssembly is not simply a newer spelling or syntax for asm.js: it is a separate format. JavaScript remains important for loading modules and connecting compiled functionality to the rest of a web application.
Is asm.js still used?
MDN’s current guidance calls the asm.js specification deprecated and suggests WebAssembly for high-performance browser code. (MDN: asm.js – Game development) That status does not mean existing asm.js programs have stopped working. The format is chiefly relevant when maintaining legacy projects, examining older Emscripten output, or understanding the history of compiled code in browsers.
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What did Firefox’s asm.js optimization do?
As a historical, browser-specific example, MDN says Gecko could compile asm.js off the main thread and cache generated machine code, beginning with Firefox 28. (MDN: <script>: The Script element) That behavior could reduce compilation work on later loads. It illustrates how engine recognition affected execution, not a feature or performance promise for current browsers generally.
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