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title: JavaScript Engines
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Browser vendors and other organizations have built "JavaScript engines". They
are independent software libraries that are capable of running JS scripts.
The most popular JavaScript engine is V8. Designed for embedding in software,
it powers Chrome, NodeJS, UXP, Deno and many other platforms.
There are many other JS engines with different design goals. Some are designed
for low-power or low-memory environments. Others aim for interoperability with
specific programming languages or environments. Typically they support ES3 and
are capable of running SheetJS code.
This demo showcases a number of JS engines and language bindings.
## General Caveats
Common browser and NodeJS APIs are often missing from light-weight JS engines.
**Global**
Some engines do not provide `globalThis` or `global` or `window`. A `global`
variable can be exposed in one line that should be run in the JS engine:
```js
var global = (function(){ return this; }).call(null);
```
**Console**
Some engines do not provide a `console` object. `console.log` can be shimmed
using the engine functionality. For example, `hermes` provides `print()`:
```js
var console = { log: function(x) { print(x); } };
```
**Binary Data**
Some engines do not provide easy ways to exchange binary data. For example, it
is common to pass null-terminated arrays, which would truncate XLSX, XLS, and
other exports. APIs that accept pointers without length should be avoided.
Base64 strings are safe for passing between JS and native code, but they should
only be used when there is no safe way to pass `ArrayBuffer` or `Uint8Array`.
## Engines
This list is sorted in alphabetical order.
### ChakraCore
ChakraCore is an embeddable JS engine written in C++.
This demo has been moved [to a dedicated page](/docs/demos/engines/chakra).
### Duktape
Duktape is an embeddable JS engine written in C. It has been ported to a number
of exotic architectures and operating systems.
This demo has been moved [to a dedicated page](/docs/demos/engines/duktape).
The demo includes examples in C and Perl.
### Goja
Goja is a pure Go implementation of ECMAScript 5.
This demo has been moved [to a dedicated page](/docs/demos/engines/goja).
### Hermes
Hermes is an embeddable JS engine for React Native. The library and binary
distributions include a command-line tool `hermes` for running JS scripts.
The simplest way to interact with the engine is to pass Base64 strings. The make
target builds a very simple payload with the data.
:::note
The official release includes the `hermes` standalone tool. While applications
should link against the official libraries, the standalone tool is useful for
verifying functionality.
:::
Complete Example (click to show)
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, shim, and test file:
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"
/* sheetjs (C) 2013-present SheetJS -- https://sheetjs.com */
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
```
Complete Example (click to show)
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) Build the library and command line tool with required options:
```bash
git clone --depth=1 https://github.com/jerryscript-project/jerryscript.git
cd jerryscript
python tools/build.py --error-messages=ON --logging=ON --mem-heap=8192 --cpointer-32bit=ON
```
1) Download the standalone script, shim, and test file:
2) Bundle the test file and create `payload.js`:
```bash
node -e "fs.writeFileSync('payload.js', 'var payload = \"' + fs.readFileSync('pres.xlsx').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); } };
```
- `jerry.js` will call `XLSX.read` and `XLSX.utils.sheet_to_csv`:
```js title="jerry.js"
/* sheetjs (C) 2013-present SheetJS -- https://sheetjs.com */
var wb = XLSX.read(payload, {type:'base64'});
console.log(XLSX.utils.sheet_to_csv(wb.Sheets[wb.SheetNames[0]]));
```
4) Create the amalgamation `xlsx.jerry.js`:
```bash
cat global.js xlsx.full.min.js payload.js jerry.js > xlsx.jerry.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 `jerry` standalone binary:
```bash
build/bin/jerry xlsx.jerry.js; echo $?
```