TUTORIAL · STEP 6 OF 7

Callbacks

A function pointer parameter takes a JavaScript function, and C++ calls it like any other.

countdown calls its Visitor once for every number, and each call runs the arrow function from main.js. applyTwice uses what its Transform returns.

Each JavaScript function handed to C++ holds a slot in the module until Module.releaseCallback(fn) frees it, so release a function once C++ no longer needs it.

Try it

Pass applyTwice a function that squares its argument and check the result.

#pragma once
 
// A function pointer parameter takes a JavaScript function,
// which C++ then calls like any other.
typedef void (*Visitor)(int value);
typedef int (*Transform)(int value);
 
void countdown(int from, Visitor visit);
int applyTwice(Transform transform, int value);
 
 
#pragma once

// A function pointer parameter takes a JavaScript function,
// which C++ then calls like any other.
typedef void (*Visitor)(int value);
typedef int (*Transform)(int value);

void countdown(int from, Visitor visit);
int applyTwice(Transform transform, int value);

Runs without a compile until you edit the C++.

Console

Press Run, or Ctrl+Enter, to run main.js.