What is the difference between asm.js and WebAssembly?

Asm.js and WebAssembly (commonly abbreviated as wasm) are technologies designed to run code efficiently in web browsers, but they have key differences in implementation and performance. Below is a comparison of these two technologies:

Asm.js:

• Concept: Asm.js is an optimized subset of JavaScript that provides assembly-like performance characteristics, enabling developers to write code approaching native performance.
• Performance: It is faster than regular JavaScript but typically slower than native code.
• Compatibility: As a subset of JavaScript, it can theoretically run on any browser supporting JavaScript.
• Development: Code is typically compiled from other languages (such as C or C++), though developers can also write asm.js code directly.
• Syntax: It uses JavaScript syntax with strict rules, such as type annotations, which allow JavaScript engines to perform more effective optimizations.
• Debugging: Debugging asm.js code can be challenging due to the generated code being difficult to read.

WebAssembly:

• Concept: WebAssembly is a new code format designed as a compilation target for web applications, enabling developers to run high-performance code on web pages.
• Performance: It is generally faster than asm.js, approaching native code performance.
• Compatibility: It is widely supported by modern web browsers and is supported by all major browsers despite being newer.
• Development: It is compiled from other languages (such as C/C++/Rust), but direct WebAssembly code writing is not supported as it uses a binary format instead of JavaScript syntax; it can be converted to a text format (WAT) using appropriate tools.
• Syntax: WebAssembly is not a text-based programming language but a binary instruction set, which makes its loading and parsing very fast.
• Debugging: WebAssembly has better debugging support compared to asm.js, though debugging can still be challenging due to its low-level encoding format.

In summary, WebAssembly is designed as a more modern and efficient solution for providing high-performance code execution across platforms. With ongoing development and improvements, it is gradually replacing asm.js.