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Internals

Bytecode​

The compiler generates bytecode directly with no intermediate representation such as a parse tree, hence it is very fast. Several optimizations passes are done over the generated bytecode.

A stack-based bytecode was chosen because it is simple and generates compact code.

For each function, the maximum stack size is computed at compile time so that no runtime stack overflow tests are needed.

A separate compressed line number table is maintained for the debug information.

Access to closure variables is optimized and is almost as fast as local variables.

Direct eval in strict mode is optimized.

Runtime​

Strings​

Strings are stored either as an 8 bit or a 16 bit array of characters. Hence random access to characters is always fast.

The C API provides functions to convert JavaScript Strings to C UTF-8 encoded strings. The most common case where the JavaScript string contains only ASCII characters involves no copying.

Objects​

The object shapes (object prototype, property names and flags) are shared between objects to save memory.

Arrays with no holes (except at the end of the array) are optimized.

TypedArray accesses are optimized.

Atoms​

Object property names and some strings are stored as Atoms (unique strings) to save memory and allow fast comparison. Atoms are represented as a 32 bit integer. Half of the atom range is reserved for immediate integer literals from 0 to 2^31-1.

Numbers​

Numbers are represented either as 32-bit signed integers or 64-bit IEEE-754 floating point values. Most operations have fast paths for the 32-bit integer case.

Garbage collection​

Reference counting is used to free objects automatically and deterministically. A separate cycle removal pass is done when the allocated memory becomes too large. The cycle removal algorithm only uses the reference counts and the object content, so no explicit garbage collection roots need to be manipulated in the C code.

JSValue​

It is a JavaScript value which can be a primitive type (such as Number, String, ...) or an Object. NaN boxing is used in the 32-bit version to store 64-bit floating point numbers. The representation is optimized so that 32-bit integers and reference counted values can be efficiently tested.

In 64-bit code, JSValue are 128-bit large and no NaN boxing is used. The rationale is that in 64-bit code memory usage is less critical.

In both cases (32 or 64 bits), JSValue exactly fits two CPU registers, so it can be efficiently returned by C functions.

Function call​

The engine is optimized so that function calls are fast. The system stack holds the JavaScript parameters and local variables.

RegExp​

A specific regular expression engine was developed. It is both small and efficient and supports all the ES2020+ features including the Unicode properties. As the JavaScript compiler, it directly generates bytecode without a parse tree.

Backtracking with an explicit stack is used so that there is no recursion on the system stack. Simple quantifiers are specifically optimized to avoid recursions.

Infinite recursions coming from quantifiers with empty terms are avoided.

The full regexp library weighs about 15 KiB (x86 code), excluding the Unicode library.

Unicode​

A specific Unicode library was developed so that there is no dependency on an external large Unicode library such as ICU. All the Unicode tables are compressed while keeping a reasonable access speed.

The library supports case conversion, Unicode normalization, Unicode script queries, Unicode general category queries and all Unicode binary properties.

The full Unicode library weighs about 45 KiB (x86 code).

BigInt​

BigInt is implemented with the libbf library. It weights about 90 KiB (x86 code) and provides arbitrary precision IEEE 754 floating point operations and transcendental functions with exact rounding.