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Dynamic Type

The dynamic type is not supported in code targeting the JVM

Being a statically typed language, Kotlin still has to interoperate with untyped or loosely typed environments, such as the JavaScript ecosystem. To facilitate these use cases, the dynamic type is available in the language:

val dyn: dynamic = ...

The dynamic type basically turns off Kotlin's type checker:

  • a value of this type can be assigned to any variable or passed anywhere as a parameter;
  • any value can be assigned to a variable of type dynamic or passed to a function that takes dynamic as a parameter;
  • null-checks are disabled for such values.

The most peculiar feature of dynamic is that we are allowed to call any property or function with any parameters on a dynamic variable:

dyn.whatever(1, "foo", dyn) // 'whatever' is not defined anywhere
dyn.whatever(*arrayOf(1, 2, 3))

On the JavaScript platform this code will be compiled "as is": dyn.whatever(1) in Kotlin becomes dyn.whatever(1) in the generated JavaScript code.

When calling functions written in Kotlin on values of dynamic type, keep in mind the name mangling performed by the Kotlin to JavaScript compiler. You may need to use the @JsName annotation to assign well-defined names to the functions that you need to call.

A dynamic call always returns dynamic as a result, so we can chain such calls freely:

dyn.foo().bar.baz()

When we pass a lambda to a dynamic call, all of its parameters by default have the type dynamic:

dyn.foo {
    x -> x.bar() // x is dynamic
}

Expressions using values of dynamic type are translated to JavaScript "as is", and do not use the Kotlin operator conventions. The following operators are supported:

  • binary: +, -, *, /, %, >, < >=, <=, ==, !=, ===, !==, &&, ||
  • unary
    • prefix: -, +, !
    • prefix and postfix: ++, --
  • assignments: +=, -=, *=, /=, %=
  • indexed access:
    • read: d[a], more than one argument is an error
    • write: d[a1] = a2, more than one argument in [] is an error

in, !in and .. operations with values of type dynamic are forbidden.

For a more technical description, see the spec document.