easy setup example https://github.com/marcuswestin/WebViewJavascriptBridge

install_checker.js

function connectWebViewJavascriptBridge(callback) {
    var rightBack = function (bridge) {
        bridge.init(function (message, responseCallback) {
            if (responseCallback) {
                responseCallback("Right back atcha")
            }
        });
        callback(bridge);
    };
    if (window.WebViewJavascriptBridge) {
        rightBack(WebViewJavascriptBridge)
    } else {
        document.addEventListener('WebViewJavascriptBridgeReady', function () {
            rightBack(WebViewJavascriptBridge)
        }, false)
    }
}
connectWebViewJavascriptBridge(function (bridge) {
    var appSchemeList = ["com.apple.safari"];
    bridge.callHandler('isInstalledApp', appSchemeList, function (responseData) {
        document.querySelector("#output").textContent = JSON.stringify(responseData, null, 4);
    })
});