Type Mappings

WasmTarget.jl maps Julia types to WasmGC types. The compiler reads the fully-inferred IR from Base.code_typed() and translates each concrete Julia type to its WASM counterpart.

Primitive Types

Julia Type	WASM Type	Notes
`Int32`, `UInt32`	`i32`	32-bit integer
`Int64`, `UInt64`, `Int`	`i64`	64-bit integer (`Int` is `Int64` on 64-bit)
`Float32`	`f32`	32-bit float
`Float64`	`f64`	64-bit float
`Bool`	`i32`	`0` or `1`

Reference Types

Julia Type	WASM Type	Notes
`String`	WasmGC `array(i32)`	One i32 per character (Unicode codepoints)
`struct Foo ... end`	WasmGC `struct`	Fields map directly
`Tuple{A, B, ...}`	WasmGC `struct`	Immutable struct
`Vector{T}`	WasmGC `struct{array, length}`	Mutable array with length
`Matrix{T}`	WasmGC `struct{array, sizes}`	Data array + size tuple
`JSValue`	`externref`	Opaque JS object reference

Struct Mapping

Julia structs become WasmGC struct types with fields in declaration order:

struct Point
    x::Float64
    y::Float64
end

Becomes a WasmGC struct type with two f64 fields. Mutable structs (mutable struct) work the same way but allow field mutation via struct.set.

Vector Mapping

Vector{T} is represented as a WasmGC struct containing:

A WasmGC array of the element type
A length field (i32)

This mirrors Julia's internal representation and allows efficient element access and length queries.

JSValue

JSValue is a primitive type that maps to WASM externref. It represents an opaque handle to a JavaScript value – a DOM element, a JS object, a function reference, etc.

# JSValue appears in function signatures for JS interop
function set_text(el::JSValue, text::Int32)::Nothing
    # Implemented as a WASM import from JS
end

See JS Interop for full details.

WasmGlobal{T, IDX}

WasmGlobal{T, IDX} is a type-safe handle for WASM global variables. The type parameter T determines the value type, and IDX is the compile-time global index. See JS Interop for usage.