The Velt Reference#

This is the precise description of the Velt language: what the compiler accepts, what it rejects, and what the compiled program does. For a guided introduction, read the Book; for imported modules, the standard library.

Velt is TypeScript compiled to native code, with Rust-class performance, a multi-core runtime and no garbage collector. Three rules shape every decision:

• valid TypeScript runs as it does in Node;
• where JavaScript's behavior is a trap (comparing 1 == "1", a forgotten await), the compiler stops and says what to write instead;
• TypeScript is the starting point, not the limit: Velt adds its own opt-in features wherever they make programs faster or safer (integer types, typed errors, shared, extend).

Conventions#

• This reference describes what the compiler does. Every code block marked as Velt is compiled by the documentation tests, and the behavior is pinned by the end-to-end tests in tests/golden/.
• Planned marks a decided design that is not built yet. Planned code is never shown as working.
• Error messages are quoted verbatim, so you can search for them.

Contents#

1. Lexical structure — programs, comments, literals, keywords, operators
2. Types — numbers, strings, equality, null, literal types, unions, discriminated unions, intersection, branded and indexed access types, enums, objects, arrays, tuples, maps
3. Variables and conditions — const/let, truthiness, module state
4. Functions and closures — parameters, generics, arrows, captures
5. Classes, structs, interfaces and generics — members, inheritance, dispatch, extend, Comparable
6. Control flow — loops, for...of, switch
7. Errors — typed throw/try/catch, throws clauses, panics
8. Async and concurrency — promises, spawn, shared, Mutex
9. Memory model — ownership, inferred mutation, exclusive access, using
10. Modules and packages — imports, exports, specifiers
11. Built-ins — what is in scope without an import
12. Diagnostics — the format and wording of compiler errors
13. TSX — JSX syntax, the provider, typing of elements and components, async components, how elements are compiled
14. System functions — declare system function: the C library, safely
15. Source generators — declare generated function: bodies a library writes per type at compile time (experimental)

How Velt differs from TypeScript, item by item, is in Velt for TypeScript developers.

Planned changes at a glance#

Decided direction that is not implemented yet. Each chapter says where it applies.