JavaScriptKit 2.0.1

JavaScriptKit 2.0.1

Maintained by Alexis Aubry Radanovic.


Build StatusVersion License Platform

JavaScriptKit is a powerful replacement for JavaScriptCore to use with your WebKit web views. Supports iOS and macOS.


  • Generate and evaluate type-safe JavaScript expressions in WKWebView
  • Automatically encode and decode values, JSON objects and enumerations to and from JavaScript
  • Easy error handling
  • Documented



To use CocoaPods, add the following to your Podfile:

pod 'JavaScriptKit', '~> 2.0'


To use Carthage, add the following to your Cartfile:

github "alexaubry/JavaScriptKit" ~> 2.0


1.0.x 2.0.x
Minimum iOS Version 8.0 8.0
Minimum macOS Version 10.10 10.10
Supported Swift Version(s) 4.0.x 4.2.x

How it works

The library is structured around the JSExpression protocol. Expressions can be represented as a JavaScript expression string, and have their return type defined at compile-time for better type safety.

You can evaluate expressions inside of a WKWebView. You provide a callback block that will be called with a Result object, containing either the value returned on success, or the error thrown by the web view on failure. Callback blocks are always executed on the main thread.

When the web view returns the result, JavaScriptKit uses a custom Decoder to decode it into the return type you specified. This allows you to set the return type to any Decodable type (structures, classes, primitive types, enumeration, array, dictionary, ...).


Get the value of a variable

Use the JSVariable expression type to get the value of a variable at the specified key path.

Example 1.1

Get the title of the current document

let titleVariable = JSVariable<String>("document.title")

webView.evaluate(expression: titleVariable) { result in
    switch result {
    case .success(let title):
        // do something with the `title` string

    case .failure(let error):
        // handle error
  • The title value provided on success is a String.

Call a function

Use the JSFunction expression type to call a function at the specified key path. You can pass as many arguments as needed. They must conform to the Encodable protocol to be converted to a JavaScript representation.

When the function does not return a value, use the JSVoid return type.

Example 2.1

URI-Encode a String

let encodeURI = JSFunction<String>("window.encodeURI", arguments: "Hello world")

webView.evaluate(expression: encodeURI) { result in
    switch result {
    case .success(let encodedURI):
        // do something with the `encodedURI` string

    case .failure(let error):
        // handle error
  • The alert expression will be converted to: "this.window.encodeURI("Hello world");".
  • The encodedURI value provided on success is a String.

Example 2.2

Show an alert

let alert = JSFunction<JSVoid>("window.alert", arguments: "Hello from Swift!")
webView.evaluate(expression: alert, completionHandler: nil)
  • The alert expression will be converted to: "this.window.alert("Hello from Swift!");".
  • To ignore the result of the expression, pass nil for the completionHandler argument.

Example 2.3

Reload the window

let reload = JSFunction<JSVoid>("location.reload")

webView.evaluate(expression: reload, completionHandler: nil)
  • You can omit the arguments parameter if the function takes no arguments.

Run your custom scripts

Use the JSScript expression type to run your custom scripts. To create custom scripts, you define a String that contains the script to run and define the return value.

The last evaluated statement in your script will be used as the return value. Do not use return at the end of the script, as it would yield an invalid value.

Example 3.1

Get the time of the day from a time string in the document

enum TimeOfDay: String, Decodable {
    case night, morning, afternoon, evening

let scriptBody = """
function getTimeOfDay(hour) {

    if (hour >= 0 && hour < 6) {
        return "night";
    } else if (hour >= 6 && hour < 12) {
        return "morning"
    } else if (hour >= 12 && hour < 18) {
        return "afternoon"
    } else if (hour >= 18 && hour > 0) {
        return "evening"


var postPublishDate = document.getElementById("publish-date").innerHTML
var hours = new Date(postPublishDate).getHours();


let script = JSScript<TimeOfDay>(scriptBody)

webView.evaluate(expression: script) { result in

    switch result {
    case .success(let timeOfDay):
        // do something with the `timeOfDay` object

    case .failure(let error):
        // handle error

  • The timeOfDay value provided on success is a case of TimeOfDay.
  • TimeOfDay is a supported return type because it implements the Decodable protocol.


Contributions are welcome and appreciated! Here's how you should submit contributions:

  • Fork and clone the repository
  • Create a new branch for your fixes (ex: git checkout -b [your branch name])
  • Get the development dependencies by running carthage bootstrap
  • Add your changes and commit them to your branch
  • Submit a PR to the master branch

If you find a bug or think a feature is missing, please submit an issue.


Alexis Aubry, [email protected] <@_alexaubry>


JavaScriptKit is available under the MIT license. See the LICENSE file for more info.