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After evolving from SpiderMonkey to TraceMonkey, JeagerMonkey sounds like a further evolution of TraceMonkey, but it's not. Started back in January 2010, JaeagerMonkey is to eventually become the JavaScript engine for Firefox 4.0.
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JaegerMonkey, from a layman's perspective. Luckily Mozilla is well on this task, and will Firefox 4.0 they aim to bring Firefox's JavaScript performance to today's standards, if not better. SpiderMonkey is no match for current generation engines in Chrome, Safari, Opera and even Internet Explorer 9! An alternate to the baseline interpreter in SpiderMonkey is sorely needed. In this era of cut-throat competition, where each browser is trying to juice it's engine for the most performance it can give, Firefox is seriously lagging behind. To understand the combination better, let us put it like- trying to go to Kanyakumari, by taking a flight to Chennai to save time, and then walking on from there! The tracing accomplished by TraceMonkey can give incredible boosts in performance, but when it doesn't work, Firefox falls back to the much slower older SpiderMonkey interpreter engine! So, for some parts of JS compiling, TM has to take help from the legacy engine, making the overall performance suffer. It gave a throttle to the legacy SpiderMonkey (SM) engine for sure, but TM isn't a standalone engine itself, it is merely a performance booster. But over time, it has become clear that TraceMonkey can only take Mozilla so far. When version 3.5 landed with major JS performance upgrades including the TraceMonkey (TM) engine, the future of Firefox's JS engine seemed to lie in more and more optimizations to the TraceMonkey engine. Mozilla's JavaScript team is on a mission to renovate the JavaScript engine for the upcoming major release of Firefox.
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Well, the good news is - they are about to. While the WebKit-based browsers are showing great improvement in JavaScript performance with V8 (for Chrome) and SquirrelFish (also known as Nitro, for Safari) engines, and even Opera is striking with their revolutionary Carakan engine - it's time for the Firefox guys to show some moves. But when the competition is concerned, it's no match for Chrome's, Safari's and Opera's JavaScript engines. Pretty well indeed it surpassed its ancestor, the SpiderMonkey engine by being 30-40 times faster in some cases. Breaking in with Firefox version 3.5, TraceMonkey was doing its job well.