Working natively with JavaScript in iOS and Android without WebView

Using JavaScript code from native code without using a WebView (sic!) can be really useful to

• introduce dynamic functionality that was updatable without need of updating the app
• shorter iteration cycles
• share code between backend and native iOS and Android apps

You can have business logic provided by a JavaScript that then controls the app that inject native functionality in the JavaScript context.

You could even write an A/B testing framework based on this approach.

iOS
iOS offers the native JavaScriptCore:
Evaluate JavaScript programs from within an app, and support JavaScript scripting of your app.

So you can evaluate a script and read its return value natively:

context.evaluateScript(myLib);
let result = context.evaluateScript("myFunction();")
let myFunction = context.objectForKeyedSubscription("myFunction");
let parametrizedResult = myFunction.invokeMethod(...)

Or you can inject a native object into the JavaScript engine:
@objc protocol MyJSProtocol: JSExport {
    var myVar: String {get set}
}

@objc class MyObject: NSObject, MyJSProtocol {
   dynamic var myVar: String
}

...
context.setObject(MyObject.self, forKeyedSubscript: "MyObject")

Documentation JavaScriptCore


Android
For Android there are methods to inject native objects into the JavaScript context in a WebView; but you always have to initiate a WebView which is pretty heavy-weight.

There are third party libs that fill this gap:

1. J2V8, a Java wrapper around Google’s V8 JavaScript engine.
2. JS Evaluator for Android, based around the native Android WebView.
3. AndroidJSCore, a Java wrapper around the WebKit JavaScriptCore engine.
4. Duktape Android, the Duktape embeddable JavaScript engine packaged for Android.

Most stable and best performing candidate proved to be J2V8.

Comparison Shopping: Searching for JavaScript Engines for Android