# asyncjs

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This module implements types and macros for writing asynchronous code for the JS backend. It provides tools for interaction with JavaScript async API-s and libraries, writing async procedures in Nim and converting callback-based code to promises.

A Nim procedure is asynchronous when it includes the `{.async.}` pragma. It should always have a `Future[T]` return type or not have a return type at all. A `Future[void]` return type is assumed by default.

This is roughly equivalent to the `async` keyword in JavaScript code.

    proc loadGame(name: string): Future[Game] {.async.} =
      # code

should be equivalent to

    async function loadGame(name) {
      // code
    }

A call to an asynchronous procedure usually needs `await` to wait for the completion of the `Future`.

    var game = await loadGame(name)

Often, you might work with callback-based API-s. You can wrap them with asynchronous procedures using promises and `newPromise`:

    proc loadGame(name: string): Future[Game] =
      var promise = newPromise() do (resolve: proc(response: Game)):
        cbBasedLoadGame(name) do (game: Game):
          resolve(game)
      return promise

Forward definitions work properly, you just need to always add the `{.async.}` pragma:

    proc loadGame(name: string): Future[Game] {.async.}

# JavaScript compatibility

Nim currently generates `async/await` JavaScript code which is supported in modern EcmaScript and most modern versions of browsers, Node.js and Electron. If you need to use this module with older versions of JavaScript, you can use a tool that backports the resulting JavaScript code, as babel.

# Imports

jsffi, macros

# Types

    Future[T] = ref object
      future*: T

  Source Edit

    PromiseJs {...}{.importcpp: "Promise".} = ref object

  Source Edit

# Procs

    proc newPromise[T](handler: proc (resolve: proc (response: T))): Future[T] {...}{.
        importcpp: "(new Promise(#))".}

  A helper for wrapping callback-based functions into promises and async procedures Source Edit

    proc newPromise(handler: proc (resolve: proc ())): Future[void] {...}{.
        importcpp: "(new Promise(#))".}

  A helper for wrapping callback-based functions into promises and async procedures Source Edit

# Macros

    macro async(arg: untyped): untyped

  Macro which converts normal procedures into javascript-compatible async procedures Source Edit