add fetch, console.log and Promise.then/catch to JavaScriptCore on iOS/Mac

JavaScriptCore+fetch.swift

import JavaScriptCore

extension JSContext {
    subscript(key: String) -> Any {
        get {
            return self.objectForKeyedSubscript(key) as Any
        }
        set{
            self.setObject(newValue, forKeyedSubscript: key as NSCopying & NSObjectProtocol)
        }
    }
}

@objc protocol JSConsoleExports: JSExport {
    static func log(_ msg: String)
}

class JSConsole: NSObject, JSConsoleExports {
    class func log(_ msg: String) {
        print(msg)
    }
}

class JSPromise: NSObject {
    enum JSPromiseResult {
        case success(Any)
        case failure(Any)
    }
    private var result: JSPromiseResult? {
        didSet {result.map(report)}
    }
    private var callbacks: [(JSPromiseResult) -> Void] = []
    func observe(using callback: @escaping (JSPromiseResult) -> Void) {
        if let result = result {
            return callback(result)
        }
        
        callbacks.append(callback)
    }
    
    private func report(result: JSPromiseResult) {
        callbacks.forEach { $0(result) }
        callbacks = []
    }
    
    convenience init(executor: @escaping (@escaping(Any)->Void, @escaping(Any)->Void) -> Void) {
        self.init()
        
        executor {[weak self] resolve in
            self?.result = .success(resolve)
        } _: {[weak self] reject in
            self?.result = .failure(reject)
        }
    }
    
    override init() {
    }
}

@objc protocol JSPromiseExports: JSExport {
    func then(_ resolve: JSValue) -> JSPromise
    func `catch`(_ reject: JSValue) -> JSPromise
}

extension JSPromise: JSPromiseExports {
    func then(_ block: JSValue) -> JSPromise {
        let weakBlock = JSManagedValue(value: block, andOwner: self)
        
        let promise = JSPromise()
        observe { result in
            switch result {
            case .success(let value):
                let next = weakBlock?.value.call(withArguments: [value]) as Any
                promise.result = .success(next)
            case .failure(let error):
                promise.result = .failure(error)
            }
        }
        
        return promise
    }
    
    func `catch`(_ block: JSValue) -> JSPromise {
        let weakBlock = JSManagedValue(value: block, andOwner: self)
        
        let promise = JSPromise()
        observe { result in
            switch result {
            case .success(let value):
                promise.result = .success(value)
            case .failure(let error):
                let next = weakBlock?.value.call(withArguments: [error]) as Any
                promise.result = .failure(next)
            }
        }
        
        return promise
    }
}

extension JSContext {
    static var plus:JSContext? {
        let jsMachine = JSVirtualMachine()
        guard let jsContext = JSContext(virtualMachine: jsMachine) else {
            return nil
        }
        
        jsContext.evaluateScript("""
            Error.prototype.isError = () => {return true}
        """)
        jsContext["console"] = JSConsole.self
        jsContext["Promise"] = JSPromise.self
        
        let fetch:@convention(block) (String) -> JSPromise = { link in
            return JSPromise{ resolve, reject in
                if let url = URL(string: link) {
                    URLSession.shared.dataTask(with: url){ (data, response, error) in
                        if let error = error {
                            reject(error.localizedDescription)
                        } else if
                            let data = data,
                            let string = String(data: data, encoding: String.Encoding.utf8) {
                            reject(string)
                        } else {
                            reject("\(url) is empty")
                        }
                    }.resume()
                } else {
                    reject("\(link) is not url")
                }
            }
        }
        
        jsContext["fetch"] = unsafeBitCast(fetch, to: JSValue.self)
        
        return jsContext
    }
}


jsContext.evaluateScript("""
fetch("https://github.com")
.then(data=>{
    console.log(data)
})
.catch(e=>console.log(e))
""")

There is some error

JS Error: Optional(TypeError: fetch("https://github.com")
.then is not a function. (In 'fetch("https://github.com")
.then(data=>{
    console.log(data)
})', 'fetch("https://github.com")
.then' is undefined))

@jigneshD11 These is not work in playground.

guard let context = JSContext.plus else {
        fatalError("Could not create JSContext")
    }

context.evaluateScript("""
(function () {
    fetch("https://jsonplaceholder.typicode.com/todos/1")
        .then((data) => {
            console.log(data);
        })
        .catch((e) => console.log(e));
})();
""")