เรียกใช้ฟังก์ชัน JavaScript จาก C/C++

So far you have called C/C++ from JavaScript. But WASM code often needs to reach back into JS, to log, manipulate the DOM, or call browser APIs.

Emscripten provides several ways for your C/C++ code to call JavaScript.

WASM is sandboxed, it cannot touch the DOM, fetch, or timers directly. Anything outside pure computation must go through JavaScript.

• Update the page
• Read user input
• Use Web APIs like localStorage

The EM_JS macro lets you define a C function whose body is JavaScript. Call it like any C function.

For one-off JS, EM_ASM runs a JavaScript snippet inline without declaring a separate function.

EM_ASM_ variants let you pass arguments and receive return values.

Strings live in WASM memory. Use Emscripten helpers like UTF8ToString to read a C string pointer in JS.

For larger interop, put JS functions in a library file and link it with --js-library.

Functions from a JS library are declared extern in C so the compiler links them.

JS can hand a C function pointer back, useful for callbacks, via Module.cwrap or function pointers passed across the boundary.

Crossing the JS/WASM boundary has overhead. Keep hot loops inside WASM and call JS only at boundaries (I/O, DOM), not inside tight inner loops.

For calling JS from C/C++:

• Use EM_JS for named JS functions, EM_ASM for snippets
• Convert pointers with UTF8ToString
• Use a JS library file for substantial interop
• Minimize boundary crossings in hot paths

Which Emscripten macro defines a C-callable function whose body is written in JavaScript?

You can now call JavaScript from your WASM code:

• EM_JS and EM_ASM embed JS in C/C++
• EM_ASM_INT passes args and returns values
• JS library files handle larger interop
• Keep boundary crossings out of hot loops

Two-way interop unlocks the full power of the browser from compiled code.