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Swift + JavaScriptCore

JavaScriptCore (JSC) is the JavaScript engine powering the Safari web browser.

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

This demo uses JSC and SheetJS to read and write spreadsheets. We'll explore how to load SheetJS in a JSC context and process spreadsheets and structured data from C++ and Swift programs.

This demo was tested in the following environments:

Swift Built-in

Swift on MacOS supports JavaScriptCore without additional dependencies.

ArchitectureSwiftDate
darwin-x646.0.22024-12-17
darwin-arm5.102024-06-30

C / C++ Compiled from Source

JavaScriptCore can be built from source and linked in C / C++ programs.

ArchitectureVersionDate
darwin-x647618.1.15.14.72024-04-24
darwin-arm7618.2.12.11.72024-05-24
linux-x647618.2.12.11.72024-06-22
linux-arm7618.2.12.11.72024-06-22

Swift Compiled from Source

Swift compiler can link against libraries built from the JavaScriptCore source.

ArchitectureVersionDate
linux-x647618.2.12.11.72024-06-22
linux-arm7618.2.12.11.72024-06-22

Integration Details​

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

Binary strings can be passed back and forth using String.Encoding.isoLatin1.

The SheetJS read method1, with the "binary" type, can parse binary strings.

The write method2, with the "binary" type, can create binary strings.

Initialize JSC​

A JSC context can be created with the JSContext function:

var context: JSContext!
do {
  context = JSContext();
  context.exceptionHandler = { _, X in if let e = X { print(e.toString()!); }; };
} catch { print(error.localizedDescription); }

JSC does not provide a global variable. It can be created in one line:

do {
  context.evaluateScript("var global = (function(){ return this; }).call(null);");
} catch { print(error.localizedDescription); }

Load SheetJS Scripts​

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

let src = try String(contentsOfFile: "xlsx.full.min.js");
context.evaluateScript(src);

To confirm the library is loaded, XLSX.version can be inspected:

let XLSX: JSValue! = context.objectForKeyedSubscript("XLSX");
if let ver = XLSX.objectForKeyedSubscript("version") { print(ver.toString()); }

Reading Files​

String(contentsOf:encoding:) reads from a path and returns an encoded string:

/* read sheetjs.xls as Base64 string */
let file_path = shared_dir.appendingPathComponent("sheetjs.xls");
let data: String! = try String(contentsOf: file_path, encoding: String.Encoding.isoLatin1);

This string can be loaded into the JS engine and processed:

/* load data in JSC */
context.setObject(data, forKeyedSubscript: "payload" as (NSCopying & NSObjectProtocol));

/* `payload` (the "forKeyedSubscript" parameter) is a binary string */
context.evaluateScript("var wb = XLSX.read(payload, {type:'binary'});");
Direct Read (click to show)

Uint8Array data can be passed directly, skipping string encoding and decoding:

let url = URL(fileURLWithPath: file)
var data: Data! = try Data(contentsOf: url);
let count = data.count;
/* Note: the operations must be performed in the closure! */
let wb: JSValue! = data.withUnsafeMutableBytes { (dataPtr: UnsafeMutableRawBufferPointer) in
  let ab: JSValue! = JSValue(jsValueRef: JSObjectMakeTypedArrayWithBytesNoCopy(context.jsGlobalContextRef, kJSTypedArrayTypeUint8Array, dataPtr.baseAddress, count, nil, nil, nil), in: context)
  /* prepare options argument */
  context.evaluateScript(String(format: "var readopts = {type:'array', dense:true}"));
  let readopts: JSValue = context.objectForKeyedSubscript("readopts");
  /* call XLSX.read */
  let XLSX: JSValue! = context.objectForKeyedSubscript("XLSX");
  let readfunc: JSValue = XLSX.objectForKeyedSubscript("read");
  return readfunc.call(withArguments: [ab, readopts]);
}

For broad compatibility with Swift versions, the demo uses the String method.

Writing Files​

When writing to binary string in JavaScriptCore, the result should be stored in a variable and converted to string in Swift:

/* write to binary string */
context.evaluateScript("var out = XLSX.write(wb, {type:'binary', bookType:'xlsx'})");

/* `out` from the script is a binary string that can be stringified in Swift */
let outvalue: JSValue! = context.objectForKeyedSubscript("out");
var out: String! = outvalue.toString();

String#write(to:atomically:encoding) writes the string to the specified path:

/* write to sheetjsw.xlsx */
let out_path = shared_dir.appendingPathComponent("sheetjsw.xlsx");
try? out.write(to: out_path, atomically: false, encoding: String.Encoding.isoLatin1);

Complete Example​

Swift​

The demo includes a sample SheetJSCore Wrapper class to simplify operations.

This demo only runs on MacOS

This example requires MacOS + Swift and will not work on Windows or Linux!

The "Swift C" section covers integration in other platforms.

  1. Ensure Swift is installed by running the following command in the terminal:
swiftc --version

If the command is not found, install Xcode.

  1. Create a folder for the project:
mkdir sheetjswift
cd sheetjswift
  1. Download the SheetJS Standalone script and the test file. Save both files in the project directory:
curl -LO https://cdn.sheetjs.com/xlsx-0.20.3/package/dist/xlsx.full.min.js
curl -LO https://docs.sheetjs.com/pres.numbers
  1. Download the Swift scripts for the demo
curl -LO https://docs.sheetjs.com/swift/SheetJSCore.swift
curl -LO https://docs.sheetjs.com/swift/main.swift
  1. Build the SheetJSwift program:
swiftc SheetJSCore.swift main.swift -o SheetJSwift
  1. Test the program:
./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.

C++​

  1. Install dependencies
Installation Notes (click to show)

The build requires CMake and Ruby.

On the Steam Deck, dependencies should be installed with pacman:

sudo pacman -Syu base-devel cmake ruby icu glibc linux-api-headers

On Debian and Ubuntu, dependencies should be installed with apt:

sudo apt-get install build-essential cmake ruby
  1. Create a project folder:
mkdir sheetjs-jsc
cd sheetjs-jsc
  1. Download and extract the WebKit snapshot:
curl -LO https://codeload.github.com/WebKit/WebKit/zip/refs/tags/WebKit-7618.2.12.11.7
mv WebKit-7618.2.12.11.7 WebKit.zip
unzip WebKit.zip
  1. Build JavaScriptCore:
cd WebKit-WebKit-7618.2.12.11.7
Tools/Scripts/build-webkit --jsc-only --cmakeargs="-DENABLE_STATIC_JSC=ON"
cd ..

When this demo was last tested on ARM64 macOS, JIT elicited runtime errors and WebAssembly elicited compile-time errors. WebAssembly and JIT must be disabled:

cd WebKit-WebKit-7618.2.12.11.7
Tools/Scripts/build-webkit --jsc-only --cmakeargs="-DENABLE_STATIC_JSC=ON" --no-jit --no-webassembly
cd ..

When this demo was tested on macOS, the build failed with the error message

Source/WTF/wtf/text/ASCIILiteral.h:65:34: error: use of undeclared identifier 'NSString'
    WTF_EXPORT_PRIVATE RetainPtr<NSString> createNSString() const;
                                 ^
1 error generated.

The referenced header file must be patched to declare NSString:

Source/WTF/wtf/text/ASCIILiteral.h (add highlighted lines)
#include <wtf/text/SuperFastHash.h>

#ifdef __OBJC__
@class NSString;
#endif

namespace WTF {

When this demo was tested on ARM64 macOS, the build failed with the error message

Source/JavaScriptCore/runtime/JSCBytecodeCacheVersion.cpp:37:10: fatal error: 'wtf/spi/darwin/dyldSPI.h' file not found
#include <wtf/spi/darwin/dyldSPI.h>
         ^~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.

The #include should be changed to a relative directive:

Source/JavaScriptCore/runtime/JSCBytecodeCacheVersion.cpp (edit highlighted lines)
#include <wtf/NeverDestroyed.h>
#include "../../WTF/wtf/spi/darwin/dyldSPI.h"
#endif
  1. Create a symbolic link to the Release folder in the source tree:
ln -s WebKit-WebKit-7618.2.12.11.7/WebKitBuild/JSCOnly/Release/ .
  1. Download sheetjs-jsc.c:
curl -LO https://docs.sheetjs.com/jsc/sheetjs-jsc.c
  1. Compile the program:
g++ -o sheetjs-jsc sheetjs-jsc.c -IRelease/JavaScriptCore/Headers -LRelease/lib -lbmalloc -licucore -lWTF -lJavaScriptCore -IRelease/JavaScriptCore/Headers
  1. Download the SheetJS Standalone script and the test file. Save both files in the project directory:
curl -LO https://cdn.sheetjs.com/xlsx-0.20.3/package/dist/xlsx.full.min.js
curl -LO https://docs.sheetjs.com/pres.numbers
  1. Run the program:
./sheetjs-jsc pres.numbers

If successful, a CSV will be printed to console. The script also tries to write to sheetjsw.xlsb, which can be opened in a spreadsheet editor.

Swift C​

For macOS and iOS deployments, it is strongly encouraged to use the official JavaScriptCore bindings. This demo is suited for Linux Swift applications.

  1. Install the Swift toolchain.8
Installation Notes (click to show)

The linux-x64 test was run on Ubuntu 22.04 using Swift 5.10.1

The linux-arm test was run on Debian 12 "bookworm" using Swift 5.10.1

  1. Follow the entire "C" demo. The shared library will be used in Swift.

  2. Enter the sheetjs-jsc folder from the previous step.

  3. Create a folder sheetjswift. It should be in the sheetjs-jsc folder:

mkdir sheetjswift
cd sheetjswift
  1. Download the SheetJS Standalone script and the test file. Save both files in the project directory:
curl -LO https://cdn.sheetjs.com/xlsx-0.20.3/package/dist/xlsx.full.min.js
curl -LO https://docs.sheetjs.com/pres.numbers
  1. Copy all generated headers to the current directory:
find ../WebKit-WebKit*/WebKitBuild/JSCOnly/Release/JavaScriptCore/Headers/  -name \*.h | xargs -I '%' cp '%' .
  1. Edit each header file and replace all instances of <JavaScriptCore/ with <. For example, JavaScript.h includes <JavaScriptCore/JSBase.h>:
JavaScript.h (original include)
#include <JavaScriptCore/JSBase.h>

This must be changed to <JSBase.h>:

JavaScript.h (modified include)
#include <JSBase.h>
  1. Print the current working directory. It will be the path to sheetjswift:
pwd
  1. Create a new header named JavaScriptCore-Bridging-Header.h :
JavaScriptCore-Bridging-Header.h
#import "/tmp/sheetjs-jsc/sheetjswift/JavaScript.h"

Replace the import path to the working directory from step 7. For example, if the path was /home/sheetjs/sheetjs-jsc/sheetjswift/, the import should be

JavaScriptCore-Bridging-Header.h
#import "/home/sheetjs/sheetjs-jsc/JavaScript.h"
  1. Create the default module map module.modulemap:
module.modulemap
module JavaScriptCore {
  header "./JavaScript.h"
  link "JavaScriptCore"
}
  1. Download SheetJSCRaw.swift:
curl -LO https://docs.sheetjs.com/swift/SheetJSCRaw.swift
  1. Build SheetJSwift:
swiftc -Xcc -I$(pwd) -Xlinker -L../WebKit-WebKit-7618.2.12.11.7/WebKitBuild/JSCOnly/Release/lib/ -Xlinker -lJavaScriptCore -Xlinker -lWTF -Xlinker -lbmalloc -Xlinker -lstdc++ -Xlinker -latomic -Xlinker -licuuc -Xlinker -licui18n -import-objc-header JavaScriptCore-Bridging-Header.h SheetJSCRaw.swift -o SheetJSwift
  1. Run the command:
./SheetJSwift pres.numbers

If successful, a CSV will be printed to console. The program also tries to write to SheetJSwift.xlsx, which can be opened in a spreadsheet editor.

Footnotes​

  1. See read in "Reading Files" ↩

  2. See writeFile in "Writing Files" ↩

  3. See JSObjectMakeTypedArrayWithBytesNoCopy in the JavaScriptCore documentation. ↩

  4. See JSObjectGetTypedArrayLength in the JavaScriptCore documentation. ↩

  5. See JSObjectGetTypedArrayBytesPtr in the JavaScriptCore documentation. ↩

  6. See read in "Reading Files" ↩

  7. See writeFile in "Writing Files" ↩

  8. See "Install Swift" in the Swift website. ↩