WebAssembly, supported by all the major browsers (i.e. from the major vendors Google, Apple, Mozilla and Microsoft), is the only alternative to JavaScript for running code in web browsers (without the help of plug-ins, such as Flash, Java or Silverlight; all being discontinued, as browsers are dropping plug-in support). Prior to its adoption, there was asm.js (a subset of JavaScript; and thus strictly works in all browsers), that's also used as a compiler target with efficient support in browsers such as Internet Explorer 11; and for such browsers that do not support WebAssembly directly, it can be compiled to asm.js and those browsers supported that way. Generally speaking programmers do not program in WebAssembly (or asm.js) directly, but use languages such as Rust, C or C++ or in theory any language, that compile to it.
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