# Types

AssemblyScript inherits WebAssembly's more specific integer, floating point and reference types:

AssemblyScript Type WebAssembly type TypeScript type Description
Integer types
i32 i32 number A 32-bit signed integer.
u32 i32 number A 32-bit unsigned integer.
i64 i64 bigint A 64-bit signed integer.
u64 i64 bigint A 64-bit unsigned integer.
isize i32 or i64 number or bigint A 32-bit signed integer in WASM32.
A 64-bit signed integer in WASM64 🦄.
usize i32 or i64 number or bigint A 32-bit unsigned integer in WASM32.
A 64-bit unsigned integer in WASM64 🦄.
Floating point types
f32 f32 number A 32-bit float.
f64 f64 number A 64-bit float.
Small integer types
i8 i32 number An 8-bit signed integer.
u8 i32 number An 8-bit unsigned integer.
i16 i32 number A 16-bit signed integer.
u16 i32 number A 16-bit unsigned integer.
bool i32 boolean A 1-bit unsigned integer.
Vector types
v128 v128 - A 128-bit vector.
Reference / GC types
ref_extern (ref extern) Object An external reference.
ref_func (ref func) Function A function reference.
ref_any (ref any) Object An internal reference. 🦄
ref_eq (ref eq) Object An equatable reference. 🦄
ref_struct (ref struct) Object A data reference. 🦄
ref_array (ref array) Array An array reference. 🦄
ref_string (ref string) string A string reference. 🦄
ref_stringview_wtf8 (ref stringview_wtf8) - A WTF-8 string view reference. 🦄
ref_stringview_wtf16 (ref stringview_wtf16) string A WTF-16 string view reference. 🦄
ref_stringview_iter (ref stringview_iter) - A string iterator reference. 🦄
Special types
void - void Indicates no return value.


The base reference types above are non-nullable. Canonical aliases, as per the spec, are available as well and refer to the respective nullable type, e.g. type externref = ref_extern | null mapping to externref == (ref null extern) in Wasm. The ref_ prefix avoids naming collisions for the time being and might be dropped in the future.

Just like in TypeScript, types are annotated after variable, function argument or class member names, separated by :, like so:

function add(a: i32, b: i32): i32 {
  let sum: i32 = a + b; // type can be inferred, annotation can be omitted
  return sum;

# Type rules

The compiler will complain when it sees an implicit conversion that might not actually be intended, quite similar to what a C compiler would do.

# Casting

In AssemblyScript, the type assertions <T>expression and expression as T known from TypeScript become explicit type conversions, essentially telling the compiler that the conversion is intended. In addition, each of the type names mentioned above, except aliases, also act as portable conversion built-ins that can be used just like i32(expression). Using portable conversions is especially useful where the exact same code is meant to be compiled to JavaScript with the TypeScript compiler (see), that otherwise would require the insertion of asm.js-style type coercions like expression | 0.

# Inference

Compared to TypeScript, type inference in AssemblyScript is limited because the type of each expression must be known in advance. This means that variable and parameter declarations must either have their type annotated or have an initializer. Without a type annotation and only an initializer, AssemblyScript will assume i32 at first and only reconsider another type if the value doesn't fit (becomes i64), is a float (becomes f64) or irrefutably has another type than these, like the type of a variable, the return type of a function or a class type. Furthermore, functions must be annotated with a return type to help the compiler make the correct decisions, for example where a literal is returned or multiple return statements are present.

# Nullability

Basic types cannot be nullable, but class and function types can. Appending | null declares a nullable type.

# Assignability

Assigning a value of one type to a target of another type can be performed without explicit casts where the full range of possible values can be represented in the target type, regardless of interpretation/signedness:

bool i8/u8 i16/u16 i32/u32 i64/u64 f32 f64

Note that isize and usize are aliases of either i32 and u32 in WASM32 respectively i64 and u64 in WASM64 🦄.

var  i8val: i8  = -128  // 0x80
var  u8val: u8  = i8val // becomes 128 (0x80)
var i16val: i16 = i8val // becomes -128 through sign-extension (0xFF80)
var u16val: u16 = i8val // becomes 65408 through masking (0xFF80)
var f32val: f32 = i8val // becomes -128.0

Wasm reference and GC types are anticipated to adhere to the following hierarchy 🦄:

Diagram of anticipated reference types hierarchy.

Dashed elements are not exposed and/or unclear. (...)-placeholders indicate the concrete subtypes, e.g. an array of a specific element type, a struct with specific field types and potentially a supertype, or a function with specific parameter and return types. For details, see Wasm GC's subtyping rules (opens new window).

# Comparability

Comparing two values of different types can be performed without an explicit cast under the same rules as outlined in assignability above

  1. if the comparison is absolute (== or ===, != or !==)
  2. if the comparison is relative (>, <, >=, <=) and both types have the same signedness

because WebAssembly has distinct operations for signed and unsigned comparisons. The comparison uses the larger type and returns bool.

Note that == and === respectively != and !== are the same in AssemblyScript because comparing two values of different types is invalid under its strict typing rules anyhow.

# Bit shifts

The result of a bit shift (<<, >>) is the left type, with the right type implicitly converted to the left type, performing an arithmetic shift if the left type is signed and a logical shift if the left type is unsigned.

The result of an unsigned right shift (>>>) is the left type (signedness is retained), with the right type implicitly converted to the left type, but always performing a logical shift.

Note that only the log2(sizeof<T>()) least significant bits of the shift affect the result:

Type Significant bits Example
i8 / u8 3 x << yx << (y & 7)
i16 / u16 4 x << yx << (y & 15)
i32 / u32 5 x << yx << (y & 31)
i64 / u64 6 x << yx << (y & 63)

If the left type is a float, an error is emitted.

# Macro types

The following macro types provide access to related types that would otherwise be impossible to obtain.

Macro type Description
native<T> Obtains the underlying native type of T, e.g. u32 if T is a class (in WASM32).
indexof<T> Obtains the index type of a collection based on the indexed access overload.
valueof<T> Obtains the value type of a collection based on the indexed access overload.
returnof<T> Obtains the return type of a function type.

# Range limits

Various range limits specific to the WebAssembly types are present as global constants for convenience:

  • const i8.MIN_VALUE: i8 = -128
    const i8.MAX_VALUE: i8 = 127
  • const i16.MIN_VALUE: i16 = -32768
    const i16.MAX_VALUE: i16 = 32767
  • const i32.MIN_VALUE: i32 = -2147483648
    const i32.MAX_VALUE: i32 = 2147483647
  • const i64.MIN_VALUE: i64 = -9223372036854775808
    const i64.MAX_VALUE: i64 = 9223372036854775807
  • const isize.MIN_VALUE: isize // WASM32: i32.MIN_VALUE, WASM64: i64.MIN_VALUE
    const isize.MAX_VALUE: isize // WASM32: i32.MAX_VALUE, WASM64: i64.MAX_VALUE
  • const u8.MIN_VALUE: u8 = 0
    const u8.MAX_VALUE: u8 = 255
  • const u16.MIN_VALUE: u16 = 0
    const u16.MAX_VALUE: u16 = 65535
  • const u32.MIN_VALUE: u32 = 0
    const u32.MAX_VALUE: u32 = 4294967295
  • const u64.MIN_VALUE: u64 = 0
    const u64.MAX_VALUE: u64 = 18446744073709551615
  • const usize.MIN_VALUE: usize = 0
    const usize.MAX_VALUE: usize // WASM32: u32.MAX_VALUE, WASM64: u64.MAX_VALUE
  • const bool.MIN_VALUE: bool = 0
    const bool.MAX_VALUE: bool = 1
  • const f32.MIN_VALUE: f32 = -3.40282347e+38
    const f32.MAX_VALUE: f32 = 3.40282347e+38
    const f32.MIN_NORMAL_VALUE: f32 = 1.17549435e-38
    const f32.MIN_SAFE_INTEGER: f32 = -16777215
    const f32.MAX_SAFE_INTEGER: f32 = 16777215
    const f32.EPSILON: f32 = 1.19209290e-07
  • const f64.MIN_VALUE: f64 = -1.7976931348623157e+308
    const f64.MAX_VALUE: f64 = 1.7976931348623157e+308
    const f64.MIN_NORMAL_VALUE: f64 = 2.2250738585072014e-308
    const f64.MIN_SAFE_INTEGER: f64 = -9007199254740991
    const f64.MAX_SAFE_INTEGER: f64 = 9007199254740991
    const f64.EPSILON: f64 = 2.2204460492503131e-16