---
title: Sharing Sheets with Hermes
sidebar_label: C++ + Hermes
description: Process structured data in C++ programs. Seamlessly integrate spreadsheets into your program by pairing Hermes and SheetJS. Handle the most complex files without breaking a sweat.
pagination_prev: demos/bigdata/index
pagination_next: solutions/input
---

import current from '/version.js';
import CodeBlock from '@theme/CodeBlock';

[Hermes](https://hermesengine.dev/) is an embeddable JS engine written in C++.

[SheetJS](https://sheetjs.com) is a JavaScript library for reading and writing
data from spreadsheets.

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

The ["Integration Example"](#integration-example) section includes a complete
command-line tool for reading data from files.

## Integration Details

:::info pass

Many Hermes functions are not documented. The explanation was verified against
commit `70af78b`.

:::

:::warning pass

The main target for Hermes is React Native.  At the time of writing, there was
no official documentation for embedding the Hermes engine in C++ programs.

:::

### Initialize Hermes

A Hermes engine instance is created with `facebook::hermes::makeHermesRuntime`:

```cpp
std::unique_ptr<facebook::jsi::Runtime> rt(facebook::hermes::makeHermesRuntime());
```

_Essential Objects_

Hermes does not expose a `console` or `global` variable, but they can be
synthesized from JS code in the runtime:

- `global` can be obtained from a reference to `this` in an unbound function:

```js
/* create global object */
var global = (function(){ return this; }).call(null);
```

- `console.log` can be constructed from the builtin `print` function:

```js
/* create a fake `console` from the hermes `print` builtin */
var console = { log: function(x) { print(x); } };
```

The code can be stored in a C string and evaluated using `prepareJavascript` to
prepare code and `evaluatePreparedJavascript` to evaluate:

```cpp
const char *init_code =
  /* create global object */
  "var global = (function(){ return this; }).call(null);"
  /* create a fake `console` from the hermes `print` builtin */
  "var console = { log: function(x) { print(x); } };"
  ;
auto src = std::make_shared<facebook::jsi::StringBuffer>(init_code);
auto js = rt->prepareJavaScript(src, std::string("<eval>"));
rt->evaluatePreparedJavaScript(js);
```

:::info Exception handling

Standard C++ exception handling patterns are used in Hermes integration code.
The base class for Hermes exceptions is `facebook::jsi::JSIException`:

```cpp
try {
  const char *init_code = "...";
  auto src = std::make_shared<facebook::jsi::StringBuffer>(init_code);
  auto js = rt->prepareJavaScript(src, std::string("<eval>"));
  rt->evaluatePreparedJavaScript(js);
} catch (const facebook::jsi::JSIException &e) {
  std::cerr << "JavaScript exception: " << e.what() << std::endl;
  return 1;
}
```

:::

### Load SheetJS Scripts

[SheetJS Standalone scripts](/docs/getting-started/installation/standalone) can
be parsed and evaluated in a Hermes context.

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

:::tip pass

There are nonstandard tricks to embed the entire script in the binary. There are
language proposals such as `#embed` (mirroring the same feature in C23).

For simplicity, the examples read the script file from the filesystem.

:::

_Reading scripts from the filesystem_

For the purposes of this demo, the standard C `<stdio.h>` methods are used:

```cpp
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;
}

// ...
  /* read SheetJS library from filesystem */
  size_t sz; char *xlsx_full_min_js = read_file("xlsx.full.min.js", &sz);
```

_Hermes Wrapper_

Hermes does not provide a friendly way to prepare JavaScript code stored in a
standard heap-allocated C string. Fortunately a wrapper can be created:

```cpp
/* Unfortunately the library provides no C-friendly Buffer classes */
class CBuffer : public facebook::jsi::Buffer {
  public:
    CBuffer(const uint8_t *data, size_t size) : buf(data), sz(size) {}
    size_t size() const override { return sz; }
    const uint8_t *data() const override { return buf; }

  private:
    const uint8_t *buf;
    size_t sz;
};

// ...
  /* load SheetJS library */
  auto src = std::make_shared<CBuffer>(CBuffer((uint8_t *)xlsx_full_min_js, sz));
```

_Evaluating SheetJS Library Code_

The code wrapper can be "prepared" with `prepareJavascript` and "evaluated" with
`evaluatePreparedJavascript`.

The second argument to `preparedJavascript` is a C++ `std::string` that holds
the source URL. Typically a name like `xlsx.full.min.js` helps distinguish
SheetJS library exceptions from other parts of the application.

```cpp
  auto js = rt->prepareJavaScript(src, std::string("xlsx.full.min.js"));
  rt->evaluatePreparedJavaScript(js);
```

_Testing_

If the library is loaded, `XLSX.version` will be a string. This string can be
pulled into the main C++ program.

The `evaluatePreparedJavascript` method returns a `facebook::jsi::Value` object
that represents the result:

```cpp
/* evaluate XLSX.version and capture the result */
auto src = std::make_shared<facebook::jsi::StringBuffer>("XLSX.version");
auto js = rt->prepareJavaScript(src, std::string("<eval>"));
facebook::jsi::Value jsver = rt->evaluatePreparedJavaScript(js);
```

The `getString` method extracts the string value and returns an internal string
object (`facebook::jsi::String`). Given that string object, the `utf8` method
returns a proper C++ `std::string` that can be printed:

```cpp
/* pull the version string into C++ code and print */
facebook::jsi::String jsstr = jsver.getString(*rt);
std::string cppver = jsstr.utf8(*rt);
std::cout << "SheetJS version " << cppver << std::endl;
```

### Reading Files

Typically C++ code will read files and Hermes will project the data in the JS
engine as an `ArrayBuffer`. SheetJS libraries can parse `ArrayBuffer` data.

Standard SheetJS operations can pick the first worksheet and generate CSV string
data from the worksheet. Hermes provides methods to convert the JS strings back
to `std::string` objects for further processing in C++.

:::note

It is strongly recommended to create a stub function to perform the entire
workflow in JS code and pass the final result back to C++.

:::

_Hermes Wrapper_

Hermes supports `ArrayBuffer` but has no simple helper to read raw memory.
Libraries are expected to implement `MutableBuffer`:

```cpp
/* ArrayBuffer constructor expects MutableBuffer */
class CMutableBuffer : public facebook::jsi::MutableBuffer {
  public:
    CMutableBuffer(uint8_t *data, size_t size) : buf(data), sz(size) {}
    size_t size() const override { return sz; }
    uint8_t *data() override { return buf; }

  private:
    uint8_t *buf;
    size_t sz;
};
```

A `facebook::jsi::ArrayBuffer` object can be created using the wrapper:

```cpp
/* load payload as ArrayBuffer */
size_t sz; char *data = read_file("pres.xlsx", &sz);
auto payload = std::make_shared<CMutableBuffer>(CMutableBuffer((uint8_t *)data, sz));
auto ab = facebook::jsi::ArrayBuffer(*rt, payload);
```

_SheetJS Operations_

In this example, the goal is to pull the first worksheet and generate CSV rows.

`XLSX.read`[^1] parses the `ArrayBuffer` and returns a SheetJS workbook object:

```js
var wb = XLSX.read(buf);
```

The `SheetNames` property[^2] is an array of the sheet names in the workbook.
The first sheet name can be obtained with the following JS snippet:

```js
var first_sheet_name = wb.SheetNames[0];
```

The `Sheets` property[^3] is an object whose keys are sheet names and whose
corresponding values are worksheet objects.

```js
var first_sheet = wb.Sheets[first_sheet_name];
```

The `sheet_to_csv` utility function[^4] generates a CSV string from the sheet:

```js
var csv = XLSX.utils.sheet_to_csv(first_sheet);
```

_C++ integration code_

:::note pass

The stub function will be passed an `ArrayBuffer` object:

```js
function(buf) {
  /* `buf` will be an ArrayBuffer */
  var wb = XLSX.read(buf);
  return XLSX.utils.sheet_to_csv(wb.Sheets[wb.SheetNames[0]]);
}
```

:::

The result after evaluating the stub is a `facebook::jsi::Value` object:

```cpp
/* define stub function to read and convert first sheet to CSV */
auto src = std::make_shared<facebook::jsi::StringBuffer>(
  "(function(buf) {"
    "var wb = XLSX.read(buf);"
    "return XLSX.utils.sheet_to_csv(wb.Sheets[wb.SheetNames[0]]);"
  "})"
);
auto js = rt->prepareJavaScript(src, std::string("<eval>"));
facebook::jsi::Value funcval = rt->evaluatePreparedJavaScript(js);
```

To call this function, the opaque `Value` must be converted to a `Function`:

```cpp
facebook::jsi::Function func = func.asObject(*rt).asFunction(*rt);
```

The `Function` exposes a `call` method to perform the function invocation. The
stub accepts an `ArrayBuffer` argument:

```cpp
/* call stub function and capture result */
facebook::jsi::Value csv = func.call(*rt, ab);
```

In the same way the library version string was pulled into C++ code, the CSV
data can be captured using `getString` and `utf8` methods:

```cpp
/* interpret as utf8 */
std::string str = csv.getString(*rt).utf8(*rt);
std::cout << str << std::endl;
```

## Complete Example

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

### Integration Example

:::note

This demo was tested in the following deployments:

| Architecture | Git Commit | Date       |
|:-------------|:-----------|:-----------|
| `darwin-x64` | `70af78b`  | 2023-08-02 |
| `darwin-arm` | `869312f`  | 2023-06-05 |

:::

0) Install build dependencies:

```bash
brew install icu4c cmake ninja
```

1) Make a project directory:

```bash
mkdir sheetjs-hermes
cd sheetjs-hermes
```

2) Download the [`Makefile`](pathname:///hermes/Makefile):

```bash
curl -LO https://docs.sheetjs.com/hermes/Makefile
```

3) Download [`sheetjs-hermes.cpp`](pathname:///hermes/sheetjs-hermes.cpp):

```bash
curl -LO https://docs.sheetjs.com/hermes/sheetjs-hermes.cpp
```

4) Build the library (this is the `init` target):

```bash
make init
```

5) Build the application:

```bash
make sheetjs-hermes
```

6) Download the standalone script and test file:

<ul>
<li><a href={`https://cdn.sheetjs.com/xlsx-${current}/package/dist/xlsx.full.min.js`}>xlsx.full.min.js</a></li>
<li><a href="https://sheetjs.com/pres.numbers">pres.numbers</a></li>
</ul>

<CodeBlock language="bash">{`\
curl -LO https://cdn.sheetjs.com/xlsx-${current}/package/dist/xlsx.full.min.js
curl -LO https://sheetjs.com/pres.numbers`}
</CodeBlock>

7) Run the application:

```bash
./sheetjs-hermes pres.numbers
```

If successful, the program will print the library version number and the
contents of the first sheet as CSV rows.

### CLI Test

:::note

This demo was last tested on 2023 May 30 against Hermes version `0.11.0`.

:::

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

0) Install the `hermes` command line tool

1) Download the standalone script and test file:

<ul>
<li><a href={`https://cdn.sheetjs.com/xlsx-${current}/package/dist/xlsx.full.min.js`}>xlsx.full.min.js</a></li>
<li><a href="https://sheetjs.com/pres.numbers">pres.numbers</a></li>
</ul>

<CodeBlock language="bash">{`\
curl -LO https://cdn.sheetjs.com/xlsx-${current}/package/dist/xlsx.full.min.js
curl -LO https://sheetjs.com/pres.numbers`}
</CodeBlock>

2) Bundle the test file and create `payload.js`:

```bash
node -e "fs.writeFileSync('payload.js', 'var payload = \"' + fs.readFileSync('pres.numbers').toString('base64') + '\";')"
```

3) Create support scripts:

- `global.js` creates a `global` variable and defines a fake `console`:

```js title="global.js"
var global = (function(){ return this; }).call(null);
var console = { log: function(x) { print(x); } };
```

- `hermes.js` will call `XLSX.read` and `XLSX.utils.sheet_to_csv`:

```js title="hermes.js"
var wb = XLSX.read(payload, {type:'base64'});
console.log(XLSX.utils.sheet_to_csv(wb.Sheets[wb.SheetNames[0]]));
```

4) Create the amalgamation `xlsx.hermes.js`:

```bash
cat global.js xlsx.full.min.js payload.js hermes.js > xlsx.hermes.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.

5) Run the script using the Hermes standalone binary:

```bash
hermes xlsx.hermes.js
```

If successful, the script will print CSV data from the test file.

[^1]: See [`read` in "Reading Files"](/docs/api/parse-options)
[^2]: See ["Workbook Object"](/docs/csf/book)
[^3]: See ["Workbook Object"](/docs/csf/book)
[^4]: See [`sheet_to_csv` in "Utilities"](/docs/api/utilities/csv#csv-output)