# surface reconstruction in javascript
**URL:** https://discourse.vtk.org/t/surface-reconstruction-in-javascript/4434
**Category:** Web
**Created:** [October 17, 2020, 10:02am UTC](https://discourse.vtk.org/t/surface-reconstruction-in-javascript/4434 "2020-10-17T10:02:25Z")
**Posts on this page:** 4
**Page:** 2
### Author:  [@Donny\_Zimmerman](https://discourse.vtk.org/u/Donny_Zimmerman)
#### Post date: [November 2, 2020, 9:24pm UTC](https://discourse.vtk.org/t/surface-reconstruction-in-javascript/4434/21 "2020-11-02T21:24:07Z")
After looking at the links you refer to, that it is a bug in Emscripten. Even if they fix the bugs, I am curious if running the wasm code in a new thread, which is required for synchronous fetch, will cause errors with VTK and OpenGL thread safety?
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### Author:  [@SArbabi](https://discourse.vtk.org/u/SArbabi)
#### Post date: [November 2, 2020, 11:15pm UTC](https://discourse.vtk.org/t/surface-reconstruction-in-javascript/4434/22 "2020-11-02T23:15:22Z")
right, when asynchronous fetching used, how can I get access to the content of the file in a variable that I want to use? fetch-\>data is accessible in downloadSucceeded, but how can I get access to that in main?
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### Author:  [@SArbabi](https://discourse.vtk.org/u/SArbabi)
#### Post date: [November 2, 2020, 11:18pm UTC](https://discourse.vtk.org/t/surface-reconstruction-in-javascript/4434/23 "2020-11-02T23:18:22Z")
regarding to your question, as [API page](https://emscripten.org/docs/api_reference/fetch.html#synchronous-fetches) suggests, I guess synchronous fetch operation is available in both main thread and pthreads with `--proxy-to-worker + -s USE_PTHREADS=1` flags.
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### Author:  [@Donny\_Zimmerman](https://discourse.vtk.org/u/Donny_Zimmerman)
#### Post date: [November 3, 2020, 4:04pm UTC](https://discourse.vtk.org/t/surface-reconstruction-in-javascript/4434/24 "2020-11-03T16:04:57Z")
I call emsripten\_fetch from a member function of my class (SwisVtkRadarWindow). I assign the reference of SwisVtkRadarWindow to the fetch attribute called “userData” by using the “this” keyword.
> emscripten\_fetch\_attr\_t attr;
> emscripten\_fetch\_attr\_init(&attr);
> strcpy(attr.requestMethod, “GET”);
> attr.attributes = EMSCRIPTEN\_FETCH\_LOAD\_TO\_MEMORY;
> attr.userData = this;
> attr.onsuccess = downloadSucceeded;
> emscripten\_fetch(&attr, path.c\_str());
Then in the downloadSucceeded callback I use a static cast to get my class from userData and process the data.
> void downloadSucceeded(emscripten\_fetch\_t\* fetch) {
>
> while (fetch-\>readyState != 4)
> {
> std::cout \<\< “Waiting For Download To Complete.” \<\< std::endl;
> }
>
> std::cout \<\< "Finished downloading " \<\< fetch-\>numBytes \<\< " bytes, and " \<\< fetch-\>totalBytes \<\< " total bytes from URL " \<\< fetch-\>url \<\< std::endl;
> // The data is now available at fetch-\>data[0] through fetch-\>data[fetch-\>numBytes-1];
> std::vector data(fetch-\>numBytes, 0);
> memcpy(data.data(), fetch-\>data, fetch-\>numBytes);
> emscripten\_fetch\_close(fetch); // Free data associated with the fetch.
>
> auto radwin = static\_cast\(fetch-\>userData);
> radwin-\>DownloadSuccess(data);
>
> }
Regarding the Emscripten compiler flags “–proxy-to-worker + -s USE\_PTHREADS=1”. When I tried this I was getting compile errors saying I can’t use --proxy-to-worker flags with USE\_PTHREADS=1, which conflicts with the documentation.
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