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ECMAScript

This folder contains the code for things mentioned directly in the ECMAScript language specification. As much as is reasonable, the structure within this folder should be similar to the specification text and code should reuse terms from the specification directly.

This is also conceptually the main entry point into the nova_vm library for embedders.

Values

The main ECMAScript Value type of Nova is found in types/language/value.rs and is a public Rust enum. The Nova Value API is thus fully open about its internal representation, and the usage of pattern matching on Value in embedders is explicitly encouraged.

Nova's Value variants always hold either an in-line stored payload directly, or "handle" to data stored on the engine heap.

Primitives

Nova's primitive types are implemented as follows.

  1. undefined and null as payload-less variants on Value.

  2. true and false as a bool carrying variant on Value.

  3. Strings are encoded as [WTF-8] and have two variants on Value: short strings stored in-line, and longer strings heap-allocated and referenced though a handle.

  4. Symbols have a single variant on Value, carrying a handle to heap-allocated data.

  5. Numbers have three variants on Value: safe integers stored in-line, double-precision floating point values with 8 trailing zeroes stored in-line, and other numbers heap-allocated and referenced through a handle.

  6. BigInts have two variants on Value: 56-bit signed values stored in-line, and larger values heap-allocated and referenced through a handle.

Objects

All non-primitive ECMAScript values are objects. Object data is always heap-allocated and referenced through a handle. Unlike most JavaScript engines, Nova does not use pointers to refer to heap-allocated data and instead chooses to use a combination of the handle's type and an index contained in the handle. This means that all object data is allocated on the heap in dedicated typed arenas, and accessing the data is done by offsetting based on the handle's contained index.

Crossreferencing

6. ECMAScript Data Types and Values

Found in the types folder.

7. Abstract Operations

Found in the abstract_operations folder.

8. Syntax-Directed Operations

This is more about the parsing so I am not sure if this needs to be in the engine at all.

If this ends up being needed then it will be in a syntax folder.

9. Executable Code and Execution Contexts

Found in the execution folder.

10. Ordinary and Exotic Objects Behaviours

Found in the builtins folder.

11-15. ECMAScript Language, and 17. Error Handling and Language Extensions

For the parts concerning evaluation of the language syntax, these are mainly found in the adjacent engine folder.

16. ECMAScript Language: Scripts and Modules

Found in the scripts_and_modules folder.

18.-28. ECMAScript Standard Built-in Objects

Found in the builtins folder.