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Version: 0.3

any

any compiles. It takes a dynamic path: each access is looked up by name and checked at run time. The typed path stays the default, and every place that uses any gets warning XS2007, so the cost is never hidden.

x.ts:2:10: warning[XS2007]: 'any' takes the dynamic path: each access is looked up by name and checked at run time (about 10x slower than a typed access)
     2 | const u: any = new User();
  help: declare a type (an interface, a class, Record<string, T>) to take the typed path

What happens to the value​

Value given to anyStored asField access
an object of a classthe same object, same layout, no extra bytesthrough a per-class table of field names
an object made as any (const o: any = { … })a Record<string, unknown>a hash lookup; more memory than a typed object
a primitivea tagged value

The Record case has its own warning text: "…and it is stored as a Record, which uses more memory than a typed object".

What works​

class User { name = "ana"; age: int = 30; greet(): string { return "hi " + this.name; } }
const u: any = new User();
console.log(u.name, u.age + 1);   // ana 31
console.log(u.greet());           // hi ana (warning: method looked up by name)
const o: any = { id: 1, tags: ["a"] };
o.extra = true;
console.log("extra" in o, JSON.stringify(o));   // true {"id":1,"tags":["a"],"extra":true}
const back: User = u;             // checked: u must be a User

Field reads and writes, x[k], k in x, arithmetic, ===, truthiness, ??, ?., templates, JSON and console.log all work. A method call on an any is looked up among the classes that declare that method.

Run-time checks​

An any meets a typed slot (a variable, a parameter, a field) through a check. A mismatch is a TypeError:

const bad: string = o.id;   // TypeError: the value is not string (Node goes on with 1)

A field that the object does not have reads as null (printed as null; Node prints undefined).

The cost​

Reading a field through any costs ten times a typed read or more. Measured on an Apple M4 (12 Nov 2026), a loop of 10 million reads: 0.4 ns per typed read, 9 ns per read through any. A typed read compiles to one load; an any read goes through a name lookup and a type check.

Keeping any out​

  • xisto build prints the count of warnings (for example xisto: 4 warnings (XS2007 4)); --warnings lists them.
  • xisto check reports them in a dynamic-path category; --details shows each place.
  • --deny-warnings turns them into errors, for CI.

Migrating off any​

  1. Run xisto check --details and start with the hottest code.
  2. Replace any with the class or interface the value really has. For bags of fields, use Record<string, T>.
  3. For a method called on an any, cast once: (x as Repo).find(id).
  4. For decoded JSON, declare the type: const t: Todo = JSON.parse(text) decodes straight into the type.

See also​