How is Javascript translated to bytecode?

Question

I can't find information on the web how W3C languages compile to machine code. I know that the gap between the web and the processor must be somehow the browser, but how does it work and what are the steps till Javascript is executed in the processor?

Links to scientific documents would be also greatly appreciated.

Answer 1

It's up to the implementation; the specification is the full description of the language and how it's supposed to work, implementations are free to satisfy that implementation in any way they like. Some implementations seem (from the outside) to run it purely as an interpreter in the old sense; others may or may not compile to bytecode; V8 compiles to machine code ( twice , for hotspots in the app).

The V8 team (V8 being the JavaScript engine in Chromium and Chrome) periodically publish descriptions of how they get the fantastic speed out of V8 that they do. You may find some of that on the project site and the Chromium blog .

Naturally, you can also kick around the code of any of the open-source implementations. V8 and SpiderMonkey (Mozilla's engine) are the two major open-source ones I know.

Answer 2

This may help : http://www.ecma-international.org/publications/standards/Ecma-262.htm

There is no spec for how to translate into bytecode (That is up to the browser developers) but there are specs about how the language should behave

Answer 3

For Firefox there's some specifications on its bytecodes:

https://developer.mozilla.org/en-US/docs/Mozilla/Projects/SpiderMonkey/Bytecodes https://developer.mozilla.org/en-US/docs/Mozilla/Projects/SpiderMonkey/Internals/Bytecode

For V8 it's compiled to native code directly:

http://jayconrod.com/posts/51/a-tour-of-v8-full-compiler

Answer 4

Javascript (as it's name suggests) is a dynamic scripting language . Meaning that it's code is analysed and executed at runtime by the web-browser's Javascript engine.

It is up to the Web-browser, how it wants to deal with Javascript. Some may generate an intermediate language, or bytecode. Some may directly analyse and execute it.

Here are the steps to the simplest way to execute Javascript (parsing and executing at runtime):

Parsing and Preprocessing ( recursive descent or otherwise)

Analysis

Execution

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Chrome's Javascript Engine compiles Javascript to native platform-specific machine code (for optimum performance) It also has a Garbage Collection Mechanism.

Answer 5

In addition to the useful, specific answers already given, the phrase 'adaptive optimisation' is probably worth looking up if performance is your main interest. JavaScript and its interpreters are just the latest instance of systems that need to convert something else to machine code at runtime, so there's plenty of wider reading. The bytecode forms of Pascal, Smalltalk, Java, etc can easily enough be viewed as an intermediate form in the process of running a defined language on arbitrary hardware — Apple's SquirrelFish explicitly creates a bytecode and uses a JIT compiler on that, for example.