The reason people call C portable assembly is because it's about as low as you can go without coding in assembly, and still have it compile to various platforms

The reason people call C portable assembly is because it's about as low as you can go without coding in assembly, and still have it compile to various platforms. You are really comparing asm.js and C as portable assembly emitters rather than targets. That said, people very occasionally use C as a target like they use asm.js, e.g. the original C++ compilers, the mars rover. asm.js is as low as you can seemingly go and have it run in a modern version of Chrome and Firefox, but those requirements prevent it from being a portable assembler in the sense that C is. In addition, it's sensible to write your program in another language first before asm.js, as I understand it. There's a whole swathe of machines (rather than users) out there that cannot run a modern version of those browsers, i.e. asm.js is not portable to them. When the code being written in and transposed from is asm.js and the destination is some other language - then asm.js will be portable assembler in the sense that C is.