How many value types does core WebAssembly have?

Seven: four number types (i32, i64, f32, f64), one vector type (v128, from the SIMD proposal), and two reference types (funcref, externref). i32 and i64 are integers with no inherent signedness; instructions decide whether a value is treated as signed or unsigned.

How does i64 map to JavaScript?

i64 maps to the JavaScript BigInt type because a 64-bit integer cannot be represented exactly by a JS Number (which is a 64-bit float with 53 bits of integer precision). Passing or returning i64 across the boundary uses BigInt values.

What is v128 and how many lanes does it have?

v128 is a 128-bit packed vector used by fixed-width SIMD. The same 128 bits are interpreted as different lane layouts: 16x i8, 8x i16, 4x i32, 2x i64, 4x f32 or 2x f64. The instruction, not the type, picks the lane interpretation.

What is the difference between funcref and externref?

funcref is an opaque reference to a WebAssembly function (used by tables and call_indirect). externref is an opaque handle to a host value such as a JavaScript object, letting Wasm hold references to host data without the host exposing its memory layout.

Are i32 and i64 signed or unsigned?

Neither — the type only fixes the bit width. The same i32 value can be read as signed or unsigned depending on the instruction (e.g. i32.div_s vs i32.div_u, or i32.lt_s vs i32.lt_u). This keeps the type system small.

WebAssembly Value Types Reference

Email me this result

Get this tool's output sent to your inbox, plus one useful tool a week. No spam, unsubscribe any time.

WebAssembly has a deliberately tiny set of value types. This reference lists each one with its single-byte binary encoding, bit width, JavaScript mapping and — for v128 — the SIMD lane layouts.

How it works

Value types appear in the binary format’s type section and in function signatures. Each is encoded as one byte:

0x7F  i32        0x7E  i64
0x7D  f32        0x7C  f64
0x7B  v128       0x70  funcref
0x6F  externref

The four number types are split into integers (i32, i64) and IEEE-754 floats (f32, f64). Integers carry no signedness — i32.div_s vs i32.div_u decides interpretation per instruction. The vector type v128 holds 128 bits reinterpreted as lanes by SIMD instructions. The two reference types are opaque handles: funcref to a Wasm function, externref to a host value.

JavaScript boundary

When Wasm functions are called from JS, types map as:

i32, f32, f64 → Number
i64 → BigInt (a 53-bit Number cannot hold 64 bits exactly)
funcref → a JS function (or null)
externref → any JS value passed through unchanged
v128 → cannot cross the JS boundary directly; it is internal to Wasm SIMD code

Keep this in mind when wiring imports and exports — passing a plain Number where an i64 is expected throws a TypeError.