src/runtime_assertions.js

#if ASSERTIONS
// Endianness check (note: assumes compiler arch was little-endian)
HEAP16[1] = 0x6373;
if (HEAPU8[2] !== 0x73 || HEAPU8[3] !== 0x63) throw 'Runtime error: expected the system to be little-endian!';

function abortFnPtrError(ptr, sig) {
#if ASSERTIONS >= 2
	var possibleSig = '';
	for(var x in debug_tables) {
		var tbl = debug_tables[x];
		if (tbl[ptr]) {
			possibleSig += 'as sig "' + x + '" pointing to function ' + tbl[ptr] + ', ';
		}
	}
	abort("Invalid function pointer " + ptr + " called with signature '" + sig + "'. Perhaps this is an invalid value (e.g. caused by calling a virtual method on a NULL pointer)? Or calling a function with an incorrect type, which will fail? (it is worth building your source files with -Werror (warnings are errors), as warnings can indicate undefined behavior which can cause this). This pointer might make sense in another type signature: " + possibleSig);
#else
	abort("Invalid function pointer " + ptr + " called with signature '" + sig + "'. Perhaps this is an invalid value (e.g. caused by calling a virtual method on a NULL pointer)? Or calling a function with an incorrect type, which will fail? (it is worth building your source files with -Werror (warnings are errors), as warnings can indicate undefined behavior which can cause this). Build with ASSERTIONS=2 for more info.");
#endif
}
#endif // ASSERTIONS