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Sheets in ChakraCore

ChakraCore is an embeddable JS engine written in C++.

SheetJS is a JavaScript library for reading and writing data from spreadsheets.

This demo uses ChakraCore and SheetJS to pull data from a spreadsheet and print CSV rows. We'll explore how to load SheetJS in a ChakraCore context and process spreadsheets from a C++ program.

The "Integration Example" section includes a complete command-line tool for reading data from files.

Integration Details​

Initialize ChakraCore​

ChakraCore provides a global object through JsGetGlobalObject:

/* initialize */
JsRuntimeHandle runtime;
JsContextRef context;
size_t cookie = 0;
JsCreateRuntime(JsRuntimeAttributeNone, nullptr, &runtime);
JsCreateContext(runtime, &context);
JsSetCurrentContext(context);

/* obtain reference to global object */
JsValueRef global;
JsGetGlobalObject(&global);

/* DO WORK HERE */

/* cleanup */
JsSetCurrentContext(JS_INVALID_REFERENCE);
JsDisposeRuntime(runtime);

Cleanup and validation code is omitted from the discussion. The integration example shows structured validation and controlled memory usage.

Load SheetJS Scripts​

SheetJS Standalone scripts can be parsed and evaluated in a ChakraCore context.

The main library can be loaded by reading the script from the file system and evaluating in the ChakraCore context:

static char *read_file(const char *filename, size_t *sz) {
  FILE *f = fopen(filename, "rb");
  if(!f) return NULL;
  long fsize; { fseek(f, 0, SEEK_END); fsize = ftell(f); fseek(f, 0, SEEK_SET); }
  char *buf = (char *)malloc(fsize * sizeof(char));
  *sz = fread((void *) buf, 1, fsize, f);
  fclose(f);
  return buf;
}

#define EVAL_FILE(path) {\
  JsValueRef filename; \
  JsValueRef result; \
  JsCreateString(path, strlen(path), &filename); \
  size_t len; const char* script = read_file(path, &len);\
  JsValueRef src;\
  JsCreateExternalArrayBuffer((void*)script, len, nullptr, nullptr, &src);\
  JsRun(src, cookie++, filename, JsParseScriptAttributeNone, &result); \
}

// ...
  /* load library */
  EVAL_FILE("shim.min.js")
  EVAL_FILE("xlsx.full.min.js")

Reading Files​

JsCreateExternalArrayBuffer can generate an ArrayBuffer from a C byte array:

/* read file */
size_t len; char *buf = read_file(argv[1], &len);

/* load data into array buffer */
JsValueRef ab;
JsCreateExternalArrayBuffer((void*)buf, len, nullptr, nullptr, &ab);

After pushing the data, it is easiest to store properties on globalThis:

/* assign to the `buf` global variable */
JsValueRef buf_str; JsCreateString("buf", strlen("buf"), &buf_str);
JsObjectSetProperty(global, buf_str, ab, true);

/* call globalThis.wb = XLSX.read(ab) */
const char* script_str ="globalThis.wb = XLSX.read(buf);"

JsValueRef script;
JsCreateExternalArrayBuffer((void*)script_str, (size_t)strlen(script_str), nullptr, nullptr, &script);
JsRun(script, cookie++, fname, JsParseScriptAttributeNone, &result);

Complete Example​

The "Integration Example" covers a traditional integration in a C application, while the "CLI Test" demonstrates other concepts using the ch CLI tool.

Integration Example​

Tested Deployments

This demo was tested in the following deployments:

ArchitectureGit CommitDate
darwin-x64c3ead3f2024-03-15
darwin-arm3a7b1202024-05-23
win10-x64c3ead3f2024-03-04
win11-arm13358c62024-07-14
linux-x641f6e17c2024-04-25
  1. Install dependencies:
brew install icu4c cmake
  1. Download ChakraCore:
git clone https://github.com/chakra-core/ChakraCore.git
cd ChakraCore
git checkout 13358c6
cd ..
  1. Build ChakraCore:
cd ChakraCore
./build.sh --static --icu=/usr/local/opt/icu4c/include --test-build -j=8
cd ..

In some test runs, the build failed with the message:

!!! couldn't find ICU ...

This was fixed with a local symlink to the icu4c folder before the build step:

cd ChakraCore
mkdir -p usr/local/opt
ln -s /opt/homebrew/opt/icu4c usr/local/opt/icu4c
cd ..
  1. Download the source file and Makefile:
curl -L -O https://docs.sheetjs.com/chakra/sheetjs.ch.cpp
curl -L -O https://docs.sheetjs.com/chakra/Makefile
  1. Build the sample application:
make

In some macOS test runs, the build failed with the message:

clang: error: no such file or directory: '/usr/local/opt/icu4c/lib/libicudata.a'

This was fixed by creating a symbolic link:

sudo mkdir -p /usr/local/opt
sudo ln -s /opt/homebrew/opt/icu4c /usr/local/opt
make
  1. Download the SheetJS Standalone script, shim script and test file. Move all three files to the project directory:
curl -L -O https://cdn.sheetjs.com/xlsx-0.20.3/package/dist/xlsx.full.min.js
curl -L -O https://cdn.sheetjs.com/xlsx-0.20.3/package/dist/shim.min.js
curl -L -O https://docs.sheetjs.com/pres.numbers
  1. Run the test program:
./sheetjs.ch pres.numbers

If successful, the program will print the contents of the first sheet as CSV.

CLI Test​

Tested Deployments

This demo was last tested on 2024-07-14 against ch commit 13358c6.

Due to limitations of the ch standalone binary, this demo will encode a test file as a Base64 string and directly add it to an amalgamated script.

  1. Download and extract the ChakraCore release ZIP. Copy the binary (bin/ch) to your project folder.

The "Integration Example" also builds the ch binary! It will typically be placed in the ChakraCore/out/Test/ folder on Linux/macOS or ChakraCore\Build\VcBuild\bin\x64_debug\ on x64 Windows.

  1. Download the SheetJS Standalone script, shim script and test file. Move all three files to the project directory:
curl -L -O https://cdn.sheetjs.com/xlsx-0.20.3/package/dist/xlsx.full.min.js
curl -L -O https://cdn.sheetjs.com/xlsx-0.20.3/package/dist/shim.min.js
curl -L -O https://docs.sheetjs.com/pres.numbers
  1. Bundle the test file and create payload.js:
node -e "fs.writeFileSync('payload.js', 'var payload = \"' + fs.readFileSync('pres.numbers').toString('base64') + '\";')"
  1. Create support scripts:
  • global.js creates a global variable:
global.js
var global = (function(){ return this; }).call(null);
  • chakra.js will call XLSX.read and XLSX.utils.sheet_to_csv:
chakra.js
var wb = XLSX.read(payload, {type:'base64'});
console.log(XLSX.utils.sheet_to_csv(wb.Sheets[wb.SheetNames[0]]));
  1. Create the amalgamation xlsx.chakra.js:
cat global.js xlsx.full.min.js payload.js chakra.js > xlsx.chakra.js

The final script defines global before loading the standalone library. Once ready, it will read the bundled test data and print the contents as CSV.

On Windows, the command should be run in WSL.

  1. Run the script using the ChakraCore standalone binary:
./ch xlsx.chakra.js

Footnotes​

  1. See "Building ChakraCore" in the ChakraCore project wiki ↩ ↩2