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Developer(s)Mozilla Foundation / Mozilla Corporation
Written inC/C++
Operating systemCross-platform
PlatformIA-32, x86-64, ARM, MIPS, SPARC[1]
TypeJavaScript engine
LicenseMPL 2.0[2]

SpiderMonkey is the code name for the first JavaScript engine, written by Brendan Eich at Netscape Communications, later released as open source and currently maintained by the Mozilla Foundation. SpiderMonkey provides JavaScript support for Mozilla Firefox and various embeddings such as the GNOME 3 desktop.


Eich "wrote JavaScript in ten days" in 1995,[3] having been "recruited to Netscape with the promise of 'doing Scheme' in the browser".[4] (The idea of using Scheme was abandoned when "engineering management [decided] that the language must ‘look like Java’".[4]) In the fall of 1996, Eich, needing to "pay off [the] substantial technical debt" left from the first year, "stayed home for two weeks to rewrite Mocha as the codebase that became known as SpiderMonkey".[3] The name SpiderMonkey may have been chosen as a reference to the movie Beavis and Butt-head Do America, in which the character Tom Anderson mentions that the title characters were "whacking off like a couple of spider monkeys."[not in citation given] [5] In 2011, Eich transferred management of the SpiderMonkey code to Dave Mandelin.[3]


Version Release date Corresponding ECMAScript version Browser version Added functionality
Old version, no longer supported: 1.0 March 1996 Netscape Navigator 2.0
Old version, no longer supported: 1.1 August 1996 Netscape Navigator 3.0
Old version, no longer supported: 1.2 June 1997 Netscape Navigator 4.0 - 4.05
Old version, no longer supported: 1.3 October 1998 ECMA-262 1st + 2nd edition Netscape Navigator 4.06-4.7x
Old version, no longer supported: 1.4 Netscape Server
Old version, no longer supported: 1.5 November 2000 ECMA-262 3rd edition Netscape Navigator 6, Firefox 1.0
Old version, no longer supported: 1.6 November 2005[6] Firefox 1.5 additional array methods, array and string generics, E4X
Old version, no longer supported: 1.7 October 2006 Firefox 2.0 iterators and generators, let statement, array comprehensions, destructuring assignment
Old version, no longer supported: 1.8 June 2008 Firefox 3.0 generator expressions, expression closures
Old version, no longer supported: 1.8.5 March 2011 ECMA-262 5th edition Firefox 4.0 JSON support
Old version, no longer supported: 1.8.8 January 2012 Firefox 10.0
Old version, no longer supported: 17 November 2012 Firefox 17.0
Old version, no longer supported: 24 September 2013 Firefox 24.0
Old version, no longer supported: 31 July 2014 Firefox 31.0
Old version, no longer supported: 38 May 2015 Firefox 38.0
Old version, no longer supported: 45 March 2016 Firefox 45.0
Old version, no longer supported: 52 March 2017 Firefox 52.0
Current stable version: 60 May 2018 Firefox 60.0


SpiderMonkey implements ECMA-262 edition 5.1 (ECMAScript) and several added features. ECMA-357 (ECMAScript for XML (E4X)) was dropped in early 2013.[7]

Even though SpiderMonkey is used in Firefox, it does not provide host environments such as Document Object Model (DOM). In Mozilla projects that support the DOM, Gecko provides the host environment.


SpiderMonkey is written in C/C++ and contains an interpreter, the IonMonkey JIT compiler, and a garbage collector.


TraceMonkey was the first JIT compiler written for the JavaScript language. The compiler was first released as part of SpiderMonkey in Firefox 3.5, providing "performance improvements ranging between 20 and 40 times faster" than the baseline interpreter in Firefox 3.[8]

Instead of compiling whole functions, TraceMonkey was a tracing JIT, which operates by recording control flow and data types during interpreter execution. This data then informed the construction of Trace Trees, highly specialized paths of native code.

Improvements to JägerMonkey eventually made TraceMonkey obsolete, especially with the development of the SpiderMonkey type inference engine. TraceMonkey is absent from SpiderMonkey from Firefox 11 onward.[9]


JägerMonkey, internally named MethodJIT, was a whole-method JIT compiler designed to improve performance in cases where TraceMonkey could not generate stable native code.[10][11] It was first released in Firefox 4 and eventually entirely supplanted TraceMonkey. It has itself been replaced by IonMonkey.

JägerMonkey operated very differently from other compilers in its class: while typical compilers worked by constructing and optimizing a control flow graph representing the function, JägerMonkey instead operated by iterating linearly forward through SpiderMonkey bytecode, the internal function representation. Although this prohibits optimizations that require instruction reordering, JägerMonkey compiling has the advantage of being very fast, which is useful for JavaScript since recompiling due to changing variable types is frequent.

Mozilla implemented a number of critical optimizations in JägerMonkey, most importantly polymorphic inline caches and type inference.[12]

The difference between TraceMonkey and JägerMonkey JIT techniques and the need for both was explained in a hacks.mozilla.org article. A more in-depth explanation of the technical details was provided by Chris Leary, one of SpiderMonkey's developers, in a blog post. More technical information can be found in other developer's blogs: dvander, dmandelin.


IonMonkey is the name of Mozilla’s current JavaScript JIT compiler, which aims to enable many new optimizations that were impossible with the prior JägerMonkey architecture.[13]

IonMonkey is a more traditional compiler: it translates SpiderMonkey bytecode into a control flow graph, using static single assignment form (SSA) for the intermediate representation. This architecture enables well-known optimizations from other programming languages to be used for JavaScript, including type specialization, function inlining, linear-scan register allocation, dead code elimination, and loop-invariant code motion.[14]

The compiler can emit fast native code translations of JavaScript functions on the ARM, x86, and x86-64 platforms. It is the default engine since Firefox 18.[15]


OdinMonkey is the name of Mozilla's new optimization module for asm.js, an easily compilable subset of JavaScript. OdinMonkey itself is not a JIT compiler, it uses the current JIT compiler. It's included with Firefox from release 22.


SpiderMonkey is intended to be embedded in other applications that provide host environments for JavaScript. An incomplete list follows:

SpiderMonkey includes a JavaScript Shell for interactive JavaScript development and for command-line invocation of JavaScript program files.[20]

See also[edit]


  1. ^ "1.8.8 - SpiderMonkey | MDN". Developer.mozilla.org. 10 January 2013. Retrieved 21 March 2013.
  2. ^ Mozilla Licensing Policies, mozilla.org, retrieved 26 March 2013
  3. ^ a b c Eich, Brendan (21 June 2011). "New JavaScript Engine Module Owner". BrendanEich.com.
  4. ^ a b Eich, Brendan (3 April 2008). "Popularity". BrendanEich.com.
  5. ^ Eich, Brendan (19 August 2011). "Mapping the Monkeysphere".
  6. ^ "New in JavaScript 1.6".
  7. ^ "759422 – Remove use of e4x in account creation". Retrieved 5 February 2013.
  8. ^ Paul, Ryan (22 August 2008). "Firefox to get massive JavaScript performance boost". Ars Technica. Retrieved 21 March 2013.
  9. ^ Nethercote, Nicholas (1 November 2011). "SpiderMonkey is on a diet | Nicholas Nethercote". Blog.mozilla.com. Retrieved 21 March 2013.
  10. ^ "JaegerMonkey – Fast JavaScript, Always! » Mystery Bail Theater". Bailopan.net. 26 February 2010. Retrieved 21 March 2013.
  11. ^ Paul, Ryan (9 March 2010). "Mozilla borrows from WebKit to build fast new JS engine". Ars Technica. Retrieved 21 March 2013.
  12. ^ "JaegerMonkey - MozillaWiki". Wiki.mozilla.org. Retrieved 21 March 2013.
  13. ^ "Platform/Features/IonMonkey - MozillaWiki". Wiki.mozilla.org. 11 February 2013. Retrieved 21 March 2013.
  14. ^ "IonMonkey: Mozilla's new JavaScript JIT compiler". Infoq.com. Retrieved 21 March 2013.
  15. ^ "Firefox Notes - Desktop". Mozilla.org. 8 January 2013. Retrieved 21 March 2013.
  16. ^ https://docs.mongodb.com/manual/release-notes/3.2-javascript
  17. ^ Bolso, Erik Inge (8 March 2005). "2005 Text Mode Browser Roundup". Linux Journal. Retrieved 5 August 2010.
  18. ^ wine-cvs mailing list, 16 September 2008: "jscript: Added regular expression compiler based on Mozilla regexp implementation"
  19. ^ "The Release Riak 0.8 and JavaScript Map/Reduce". Retrieved 24 April 2011.
  20. ^ "Introduction to the JavaScript shell". MDN. Mozilla Developer Network. 29 September 2010. Retrieved 14 December 2010. The JavaScript shell is a command-line program included in the SpiderMonkey source distribution. [...] You can use it as an interactive shell [...] You can also pass in, on the command line, a JavaScript program file to run [...]

External links[edit]