---
sidebar_position: 18
title: JavaScript Engines
---
import current from '/version.js';
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The most popular JavaScript engine is V8. Designed for embedding in software,
it powers Chrome, NodeJS, UXP, Deno and many other platforms and runtimes.
There are many other runtimes 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 a
superset of ES3 and are capable of running SheetJS code.
## 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 of marshalling binary data. For example,
it is common to pass null-terminated arrays, which would truncate XLSX and XLS
files. 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`.
## Duktape
Duktape is an embeddable JS engine written in C. It has been ported to a number
of exotic architectures and operating systems.
**Reading data**
Duktape supports `Buffer` natively but should be sliced before processing:
```c
/* parse a C char array as a workbook object */
duk_push_external_buffer(ctx);
duk_config_buffer(ctx, -1, buf, len);
duk_put_global_string(ctx, "buf");
duk_eval_string_noresult("workbook = XLSX.read(buf.slice(0, buf.length), {type:'buffer'});");
```
**Writing data**
`duk_get_buffer_data` can pull `Buffer` object data into the C code:
```c
/* write a workbook object to a C char array */
duk_eval_string(ctx, "XLSX.write(workbook, {type:'array', bookType:'xlsx'})");
duk_size_t sz;
char *buf = (char *)duk_get_buffer_data(ctx, -1, sz);
duk_pop(ctx);
```
Complete Example (click to show)
:::note
This demo was tested on macOS x64.
:::
0) Download and extract the latest release (2.7.0 at the time of writing)
```bash
curl -LO https://duktape.org/duktape-2.7.0.tar.xz
tar -xJf duktape-2.7.0.tar.xz
mv duktape-2.7.0/src/*.{c,h} .
```
1) Download the standalone script, shim and test file:
2) Save the following script to `sheetjs.duk.c`:
```c title="sheetjs.duk.c"
/* sheetjs (C) 2013-present SheetJS -- http://sheetjs.com */
#include
#include
#include
#include "duktape.h"
#define FAIL_LOAD { \
duk_push_undefined(ctx); \
perror("Error in load_file"); \
return 1; \
}
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;
}
static duk_int_t eval_file(duk_context *ctx, const char *filename) {
size_t len; char *buf = read_file(filename, &len);
if(!buf) FAIL_LOAD
duk_push_lstring(ctx, (const char *)buf, (duk_size_t)len);
duk_int_t retval = duk_peval(ctx);
duk_pop(ctx);
return retval;
}
static duk_int_t load_file(duk_context *ctx, const char *filename, const char *var) {
size_t len; char *buf = read_file(filename, &len);
if(!buf) FAIL_LOAD
duk_push_external_buffer(ctx);
duk_config_buffer(ctx, -1, buf, len);
duk_put_global_string(ctx, var);
return 0;
}
static duk_int_t save_file(duk_context *ctx, const char *filename, const char *var) {
duk_get_global_string(ctx, var);
duk_size_t sz;
char *buf = (char *)duk_get_buffer_data(ctx, -1, &sz);
if(!buf) return 1;
FILE *f = fopen(filename, "wb"); fwrite(buf, 1, sz, f); fclose(f);
return 0;
}
#define FAIL(cmd) { \
printf("error in %s: %s\n", cmd, duk_safe_to_string(ctx, -1)); \
duk_destroy_heap(ctx); \
return res; \
}
#define DOIT(cmd) duk_eval_string_noresult(ctx, cmd);
int main(int argc, char *argv[]) {
duk_int_t res = 0;
/* initialize */
duk_context *ctx = duk_create_heap_default();
/* duktape does not expose a standard "global" by default */
DOIT("var global = (function(){ return this; }).call(null);");
/* load library */
res = eval_file(ctx, "shim.min.js");
if(res != 0) FAIL("shim load")
res = eval_file(ctx, "xlsx.full.min.js");
if(res != 0) FAIL("library load")
/* get version string */
duk_eval_string(ctx, "XLSX.version");
printf("SheetJS library version %s\n", duk_get_string(ctx, -1));
duk_pop(ctx);
/* read file */
res = load_file(ctx, argv[1], "buf");
if(res != 0) FAIL("file load")
printf("Loaded file %s\n", argv[1]);
/* parse workbook */
DOIT("wb = XLSX.read(buf.slice(0, buf.length), {type:'buffer'});");
DOIT("ws = wb.Sheets[wb.SheetNames[0]]");
/* print CSV */
duk_eval_string(ctx, "XLSX.utils.sheet_to_csv(ws)");
printf("%s\n", duk_get_string(ctx, -1));
duk_pop(ctx);
/* write file */
#define WRITE_TYPE(BOOKTYPE) \
DOIT("newbuf = (XLSX.write(wb, {type:'array', bookType:'" BOOKTYPE "'}));");\
res = save_file(ctx, "sheetjsw." BOOKTYPE, "newbuf");\
if(res != 0) FAIL("save sheetjsw." BOOKTYPE)
WRITE_TYPE("xlsb")
/* cleanup */
duk_destroy_heap(ctx);
return res;
}
```
3) Compile standalone `sheetjs.duk` binary
```bash
gcc -std=c99 -Wall -osheetjs.duk sheetjs.duk.c duktape.c -lm
```
4) Run the demo:
```bash
./sheetjs.duk pres.numbers
```
If the program succeeded, the CSV contents will be printed to console and the
file `sheetjsw.xlsb` will be created. That file can be opened with Excel.
Complete Example (click to show)
0) Install Go
1) Create a `go.mod` file and install dependencies:
```bash
go mod init SheetGoja
go get github.com/dop251/goja
```
2) Download the standalone script and the shim:
3) Save the following code to `SheetGoja.go`:
```go title="SheetGoja.go"
package main
import (
b64 "encoding/base64"
"fmt"
"os"
"io/ioutil"
"github.com/dop251/goja"
)
func safe_run_file(vm *goja.Runtime, file string) {
data, err := ioutil.ReadFile(file)
if err != nil { panic(err) }
src := string(data)
_, err = vm.RunString(src)
if err != nil { panic(err) }
}
func eval_string(vm *goja.Runtime, cmd string) goja.Value {
v, err := vm.RunString(cmd)
if err != nil { panic(err) }
return v
}
func write_type(vm *goja.Runtime, t string) {
b64str := eval_string(vm, "XLSX.write(wb, {type:'base64', bookType:'" + t + "'})")
buf, err := b64.StdEncoding.DecodeString(b64str.String());
if err != nil { panic(err) }
err = ioutil.WriteFile("sheetjsg." + t, buf, 0644)
if err != nil { panic(err) }
}
func main() {
vm := goja.New()
/* initialize */
eval_string(vm, "if(typeof global == 'undefined') global = (function(){ return this; }).call(null);")
/* load library */
safe_run_file(vm, "shim.min.js")
safe_run_file(vm, "xlsx.full.min.js")
/* get version string */
v := eval_string(vm, "XLSX.version")
fmt.Printf("SheetJS library version %s\n", v)
/* read file */
data, err := ioutil.ReadFile(os.Args[1])
if err != nil { panic(err) }
vm.Set("buf", vm.ToValue(vm.NewArrayBuffer(data)))
fmt.Printf("Loaded file %s\n", os.Args[1])
/* parse workbook */
eval_string(vm, "wb = XLSX.read(buf, {type:'buffer'});")
fmt.Printf("Parsed %s\n", os.Args[1])
eval_string(vm, "ws = wb.Sheets[wb.SheetNames[0]]")
fmt.Printf("Grabbed %s\n", os.Args[1])
/* print CSV */
csv := eval_string(vm, "XLSX.utils.sheet_to_csv(ws)")
fmt.Printf("%s\n", csv)
/* write file */
write_type(vm, "csv")
}
```
4) Build `SheetGoja`:
```bash
go build SheetGoja.go
```
For testing, download and run
```bash
./SheetGoja pres.numbers
```
This will print the contents as a CSV to screen AND write to `sheetjsg.csv`
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 -- http://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 hardcoded test file and print the contents as CSV.
5) Run the script using the Hermes standalone binary:
```
hermes xlsx.hermes.js
```
Complete Example (click to show)
:::caution This demo only runs on macOS
This example requires macOS + Swift and will not work on Windows or Linux!
:::
0) Ensure Xcode is installed
1) Download the standalone script, the shim and the test file:
2) Download the Swift scripts for the demo
- [`SheetJSCore.swift`](pathname:///swift/SheetJSCore.swift) Wrapper library
- [`main.swift`](pathname:///swift/main.swift) Command-line script
3) Build the `SheetJSwift` binary:
```bash
swiftc SheetJSCore.swift main.swift -o SheetJSwift
```
4) Test the program:
```bash
./SheetJSwift pres.numbers
```
If successful, a CSV will be printed to console. The script also tries to write
to `SheetJSwift.xlsx`. That file can be verified by opening in Excel / Numbers.
Complete Example (click to show)
0) Ensure `quickjs` command line utility is installed
1) Download the standalone script, the shim and the test file:
2) Save the following script to `SheetJSQuick.js`:
```js title="SheetJSQuick.js
/* sheetjs (C) 2013-present SheetJS -- http://sheetjs.com */
/* load XLSX */
import * as std from "std";
globalThis.global = globalThis;
std.loadScript("xlsx.full.min.js");
/* read contents of file */
var rh = std.open("pres.numbers", "rb");
rh.seek(0, std.SEEK_END);
var sz = rh.tell();
var ab = new ArrayBuffer(sz);
rh.seek();
rh.read(ab, 0, sz);
rh.close();
/* parse file */
var wb = XLSX.read(ab, {type: 'array'});
/* write array */
var out = XLSX.write(wb, {type: 'array'});
/* write contents to file */
var wh = std.open("SheetJSQuick.xlsx", "wb");
wh.write(out, 0, out.byteLength);
wh.close();
```
3) Test the program:
```bash
quickjs SheetJSQuick.js
```
If successful, the script will generate `SheetJSQuick.xlsx`.
Complete Example (click to show)
0) Download the appropriate Rhino build and rename to `rhino.jar`
1) Download [`SheetJSRhino.zip`](pathname:///rhino/SheetJSRhino.zip) and unzip
2) Save the following code to `SheetJSRhino.java`:
```java title="SheetJSRhino.java"
/* sheetjs (C) 2013-present SheetJS -- http://sheetjs.com */
/* vim: set ts=2: */
import com.sheetjs.SheetJS;
import com.sheetjs.SheetJSFile;
import com.sheetjs.SheetJSSheet;
public class SheetJSRhino {
public static void main(String args[]) throws Exception {
try {
SheetJS sjs = new SheetJS();
/* open file */
SheetJSFile xl = sjs.read_file(args[0]);
/* get sheetnames */
String[] sheetnames = xl.get_sheet_names();
System.err.println(sheetnames[0]);
/* convert to CSV */
SheetJSSheet sheet = xl.get_sheet(0);
String csv = sheet.get_csv();
System.out.println(csv);
} catch(Exception e) {
throw e;
} finally {
SheetJS.close();
}
}
}
```
3) Assemble `SheetJS.jar` from the demo code:
```bash
javac -cp .:rhino.jar SheetJSRhino.java
jar -cf SheetJS.jar SheetJSRhino.class com/sheetjs/*.class
```
4) Download and test:
```bash
java -cp .:SheetJS.jar:rhino.jar SheetJSRhino pres.xlsx
```
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) Download and extract the ChakraCore release ZIP. Copy the binary (`bin/ch`)
to your project folder.
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:
```js title="global.js"
var global = (function(){ return this; }).call(null);
```
- `chakra.js` will call `XLSX.read` and `XLSX.utils.sheet_to_csv`:
```js title="chakra.js"
/* sheetjs (C) 2013-present SheetJS -- http://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.chakra.js`:
```bash
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 hardcoded test file and print the contents as CSV.
5) Run the script using the ChakraCore standalone binary:
```
./ch xlsx.chakra.js
```
Complete Example (click to show)
0) Ensure `jjs` is available on system path
1) Download the standalone script, the shim and the test file:
2) Save the following script to `SheetJSNashorn.js`:
```js title="SheetJSNashorn.js"
/* sheetjs (C) 2013-present SheetJS -- http://sheetjs.com */
/* load module */
var global = (function(){ return this; }).call(null);
load('xlsx.full.min.js');
/* helper to convert byte array to plain JS array */
function b2a(b) {
var out = new Array(b.length);
for(var i = 0; i < out.length; i++) out[i] = (b[i] < 0 ? b[i] + 256 : b[i]);
return out;
}
function process_file(path) {
java.lang.System.out.println(path);
/* read file */
var path = java.nio.file.Paths.get(path);
var bytes = java.nio.file.Files.readAllBytes(path);
var u8a = b2a(bytes);
/* read data */
var wb = XLSX.read(u8a, {type:"array"});
/* get first worksheet as an array of arrays */
var ws = wb.Sheets[wb.SheetNames[0]];
var js = XLSX.utils.sheet_to_json(ws, {header:1});
/* print out every line */
js.forEach(function(l) { java.lang.System.out.println(JSON.stringify(l)); });
}
process_file('pres.numbers');
```
3) Test the script:
```bash
jjs SheetJSNashorn.js
```
It will print out the first worksheet contents.