Concepts

Strict TypeScript syntax over WebAssembly semantics — plus closures, iteration, and generators, which upstream AssemblyScript never shipped.

Strictness

There is no any, no undefined, and no union types except nullable references. Conversions are explicit — x as u8, f as i32 (float-to-int truncates, traps out of range). Equality is two operators: == is value equality (strings compare contents), === is identity. Truthiness is JS-like: 0, NaN, null, and "" are falsy.

Template literals

Template strings interpolate, and every piece stringifies through its type’s toString: primitives via their wrapper classes (I32, F64, Bool, …, honoring their declared radix defaults), classes directly or through virtual stubs — subclass overrides dispatch correctly — and interfaces through dispatch stubs. Pieces chain through String.__concat, with the accumulator kept in a managed local so intermediates stay rooted under both GC runtimes.

declare function print(s: string): void;

export function demo(): void {
    let name = "rasc";
    let runs = 3;
    print(`compiled ${name} ${runs} times`);   // nested templates work too
}

What cannot appear inside ${} errors explicitly rather than guessing: nullable values, value classes, function values, v128, void, and classes with no (or a wrongly-typed) toString.

Program structure

• Whole-program compilation from the entry file; only what its exports reach is compiled. Entry-file exports become wasm exports.
• Top-level statements run in the wasm start function. Library global initializers run first, so the allocator is live before user code; in Rive projects, top-level statements of all files run in start — that is what executes imported scripts’ registration.
• Relative imports resolve against the importing file; bare specifiers against --lib. Cyclic imports work.

The extensions get their own chapters: Classes & Generics covers reference classes, interfaces, and monomorphized generics; Closures & Iteration the native closures, for..of, spread arguments, and generators upstream never shipped; and Decorators the annotation surface.