What JavaScript value corresponds to NULL?

[bottle2]

consider this function from libxlsxwriter:

lxw_error worksheet_write_string(lxw_worksheet *worksheet, lxw_row_t row, lxw_col_t col, const char *string, lxw_format *format);

now imagine I'm inside EM_JS and I have somewhere:

        const ptr = stringToNewUTF8(cells[j].firstElementChild.value);
        _worksheet_write_string(ws, i, j++, ptr, NULL);
        _free(ptr);

but what should I pass instead of NULL, in a way that is reliable, that is, won't break in the future, the correct way?

• 0
• null
• undefined
• some special value defined by Emscripten?

please also adding this information in https://emscripten.org/docs/porting/connecting_cpp_and_javascript/Interacting-with-code.html#interacting-with-code-direct-function-calls

[sbc100]

We don't define any special JS value for NULL.

The number 0 is probably your best bet, although null and undefined will both work since they becoming the number 0 when passed to a wasm function expecting i32 (which is the wasm type the represents a pointer).

[bottle2]

thx! fair enough! I think that out of paranoia of one day having to revise thousands of bare 0s lying around in codebase, I'll make a wrapper for 0 for now :P

[hoodmane]

Only reason I can think of is that on wasm64 targets you'll need 0n instead.

[sbc100]

In most cases we try to paper over that using automatic wrappers that covert pointers to numbers at the JS boundary (basically hiding the fact the pointers are i64 where possible)