In order to pass any other values, you need to convert them manually via corresponding APIs. Like WebAssembly itself, it has a limitation of supporting only numeric arguments and return values. EM_JS in Emscripten lets you declare a C/C++ function that is implemented by a JavaScript snippet. The EM_JS macro lets you move fetch_json to JavaScript. What if you could move fetch_json to JavaScript and reduce the overhead of intermediate steps at the same time? EM_JS macro #
#Java script snippets driver
C++ code is acting only as a driver for a series of JavaScript operations. Such code doesn't need anything from C++. Return the converted result to C++, and C++ finally reads it back.Įach await() also has to pause the C++ side by unwinding the entire call stack of the WebAssembly module, returning to JavaScript, waiting, and restoring the WebAssembly stack when the operation is complete.Convert the result from JavaScript to intermediate format.Go to JavaScript, read and convert arguments into JavaScript values.Convert C++ values passed as arguments into some intermediate format.Each operation on val needs to perform the following steps: This code works well, but it performs lots of intermediate steps. Ask to read the response body as JSON and await the returned `Promise`. Invoke fetch and await the returned `Promise`. Thread_local const val fetch = val :: global ( "fetch" ) Get and cache a binding to the global `fetch` API in each thread. await() to fetch and parse some JSON: # include
It can invoke global APIs, bind JavaScript values to C++ instances, and convert values between C++ and JavaScript types. The emcripten::val class is provided by Embind.
#Java script snippets how to
In this post you’ll learn how to use all of them for similar tasks. -js-library for advanced scenarios where you want to declare lots of JavaScript functions together as a single library.EM_ASM for embedding short snippets and executing them inline, without declaring a function.EM_ASYNC_JS that's similar to EM_JS, but makes it easier to embed asynchronous JavaScript snippets.EM_JS for embedding JavaScript snippets and binding them as C/C++ functions.emscripten::val for storing and operating on JavaScript values in C++.For asynchronous APIs, you might also need to await promises in your synchronous C/C++ code with Asyncify and read the result once the operation is finished.Įmscripten provides several tools for such interactions: You then need a way to store the values and object instances those APIs return, and a way to pass those stored values to other APIs later. When working on WebAssembly integration with the web, you need a way to call out to external APIs such as web APIs and third-party libraries.