SpiderMonkey is the JavaScript and WebAssembly implementation library of the Mozilla Firefox web browser. The implementation behaviour is defined by the ECMAScript and WebAssembly specifications.

Much of the internal technical documentation of the engine can be found throughout the source files themselves by looking for comments labelled with [SMDOC]. Information about the team, our processes, and about embedding SpiderMonkey in your own projects can be found at https://spidermonkey.dev.

Components of SpiderMonkey

🧹 Garbage Collector

JavaScript is a garbage collected language and at the core of SpiderMonkey we manage a garbage-collected memory heap. Elements of this heap have a base C++ type of gc::Cell. Each round of garbage collection will free up any Cell that is not referenced by a root or another live Cell in turn.

See GC overview for more details.

📦 JS::Value and JSObject

JavaScript values are divided into either objects or primitives (Undefined, Null, Boolean, Number, BigInt, String, or Symbol). Values are represented with the JS::Value type which may in turn point to an object that extends from the JSObject type. Objects include both plain JavaScript objects and exotic objects representing various things from functions to ArrayBuffers to HTML Elements and more.

Most objects extend NativeObject (which is a subtype of JSObject) which provides a way to store properties as key-value pairs similar to a hash table. These objects hold their values and point to a Shape that represents the set of keys. Similar objects point to the same Shape which saves memory and allows the JITs to quickly work with objects similar to ones it has seen before. See the [SMDOC] Shapes comment for more details.

C++ (and Rust) code may create and manipulate these objects using the collection of interfaces we traditionally call the JSAPI.

🗃️ JavaScript Parser

In order to evaluate script text, we parse it using the Parser into an Abstract Syntax Tree (AST) temporarily and then run the BytecodeEmitter (BCE) to generate Bytecode and associated metadata. We refer to this resulting format as Stencil and it has the helpful characteristic that it does not utilize the Garbage Collector. The Stencil can then be instantiated into a series of GC Cells that can be mutated and understood by the execution engines described below.

Each function as well as the top-level itself generates a distinct script. This is the unit of execution granularity since functions may be set as callbacks that the host runs at a later time. There are both ScriptStencil and js::BaseScript forms of scripts.

By default, the parser runs in a mode called syntax or lazy parsing where we avoid generating full bytecode for functions within the source that we are parsing. This lazy parsing is still required to check for all early errors that the specification describes. When such a lazily compiled inner function is first executed, we recompile just that function in a process called delazification. Lazy parsing avoids allocating the AST and bytecode which saves both CPU time and memory. In practice, many functions are never executed during a given load of a webpage so this delayed parsing can be quite beneficial.

⚙️ JavaScript Interpreter

The bytecode generated by the parser may be executed by an interpreter written in C++ that manipulates objects in the GC heap and invokes native code of the host (eg. web browser). See [SMDOC] Bytecode Definitions for descriptions of each bytecode opcode and js/src/vm/Interpreter.cpp for their implementation.

⚡ JavaScript JITs

In order to speed up execution of bytecode, we use a series of Just-In-Time (JIT) compilers to generate specialized machine code (eg. x86, ARM, etc) tailored to the JavaScript that is run and the data that is processed.

As an individual script runs more times (or has a loop that runs many times) we describe it as getting hotter and at certain thresholds we tier-up by JIT-compiling it. Each subsequent JIT tier spends more time compiling but aims for better execution performance.

Baseline Interpreter

The Baseline Interpreter is a hybrid interpreter/JIT that interprets the bytecode one opcode at a time, but attaches small fragments of code called Inline Caches (ICs) that rapidly speed-up executing the same opcode the next time (if the data is similar enough). See the [SMDOC] JIT Inline Caches comment for more details.

Baseline Compiler

The Baseline Compiler use the same Inline Caches mechanism from the Baseline Interpreter but additionally translates the entire bytecode to native machine code. This removes dispatch overhead and does minor local optimizations. This machine code still calls back into C++ for complex operations. The translation is very fast but the BaselineScript uses memory and requires mprotect and flushing CPU caches.


The WarpMonkey JIT replaces the former IonMonkey engine and is the highest level of optimization for the most frequently run scripts. It is able to inline other scripts and specialize code based on the data and arguments being processed.

We translate the bytecode and Inline Cache data into a Mid-level Intermediate Representation (Ion MIR) representation. This graph is transformed and optimized before being lowered to a Low-level Intermediate Representation (Ion LIR). This LIR performs register allocation and then generates native machine code in a process called Code Generation.

The optimizations here assume that a script continues to see data similar what has been seen before. The Baseline JITs are essential to success here because they generate ICs that match observed data. If after a script is compiled with Warp, it encounters data that it is not prepared to handle it performs a bailout. The bailout mechanism reconstructs the native machine stack frame to match the layout used by the Baseline Interpreter and then branches to that interpreter as though we were running it all along. Building this stack frame may use special side-table saved by Warp to reconstruct values that are not otherwise available.

🟪 WebAssembly

In addition to JavaScript, the engine is also able to execute WebAssembly (WASM) sources.

WASM-Baseline (RabaldrMonkey)

This engine performs fast translation to machine code in order to minimize latency to first execution.

WASM-Ion (BaldrMonkey)

This engine translates the WASM input into same MIR form that WarpMonkey uses and uses the IonBackend to optimize. These optimizations (and in particular, the register allocation) generate very fast native machine code.


This experimental alternative to BaldrMonkey is an optimizing WASM compiler written in Rust. This currently is used on ARM64-based platforms (which do not support BaldrMonkey).