JavaScriptCore

Source Repository

The skip-script framework is available at https://github.com/skiptools/skip-script.git ↗, which can be checked out and tested with skip test once Skip is installed.

SkipScript provides a unified interface to the JavaScriptCore script engine on both iOS (using the platform-provided JavaScriptCore libraries) and on Android (using the bundled libjsc.so library). SkipScript enables a single scripting language (JavaScript) to be embedded in a dual-platform Skip app and provide the exact same behavior on both platforms.

Setup

To include this framework in your project, add the following dependency to your Package.swift file:

let package = Package(
    name: "my-package",
    products: [
        .library(name: "MyProduct", targets: ["MyTarget"]),
    ],
    dependencies: [
        .package(url: "https://source.skip.tools/skip-script.git", from: "1.0.0"),
    ],
    targets: [
        .target(name: "MyTarget", dependencies: [
            .product(name: "SkipScript", package: "skip-script")
        ])
    ]
)

Usage

Note

Note that SkipScript will automatically be imported when it is included as a dependency and a Swift source file imports the JavaScriptCore framework.

In this case, a subset of the the Objective-C JavaScriptAPI is mimicked on the Kotlin side, passing the calls through to the underlying C interface to the JavaScriptCore API using JNA and SkipFFI.

An example of evaluating some JavaScript:

import SkipScript

let ctx = try JSContext()
let num = ctx.evaluateScript("1 + 2.3")
assert(num.toDouble() == 3.3)

NOTE: JIT compilation is blocked on iOS without a special entitlement, which can drastically impact the performance of JavaScriptCore on iOS compared to either macOS or Android (where JIT is not blocked).

Implementation

On iOS and other Darwin platforms, the built-in JavaScriptCore libraries will be used.

Android, on the other hand, does not ship JSC as part of the operating system, and so the dependency on the Android side will utilize the org.webkit:android-jsc package to bundle a native build of JavaScriptCore with the app itself. This will increase the total Android bundle size by between 5-10Mb.

Building

This project is a Swift Package Manager module that uses the Skip plugin to transpile Swift into Kotlin.

Building the module requires that Skip be installed using Homebrew ↗ with brew install skiptools/skip/skip. This will also install the necessary build prerequisites: Kotlin, Gradle, and the Android build tools.

Testing

The module can be tested using the standard swift test command or by running the test target for the macOS destination in Xcode, which will run the Swift tests as well as the transpiled Kotlin JUnit tests in the Robolectric Android simulation environment.

Parity testing can be performed with skip test, which will output a table of the test results for both platforms.