1. Add these plugins to your Ionic project:

cordova plugin add cordova-plugin-inappbrowser
cordova plugin add cordova-plugin-whitelist

2. Install ng-cordova-oauth.

Link: ng-cordova-oauth.

3. Use my fitBitService.js file:

// fitBitService.js
app.service('fitBitLoginService', function ($http, $q, $cordovaOauthUtility) {


	/*
	 * Sign into the fitbit service
	 *
	 * @param    string clientId
	 * @param    array appScope
	 * @param    object options
	 * @return   promise
	 */
	this.oauthfitbit = function (clientId, appScope, options) {

		var deferred = $q.defer();
		if (window.cordova) {
			var redirect_uri = "https://tinyurl.com/4g70";
			if (options !== undefined) {
				if (options.hasOwnProperty("redirect_uri")) {
					//   redirect_uri = options.redirect_uri;
				}
			}
			var flowUrl = "https://www.fitbit.com/oauth2/authorize?response_type=token&client_id="
				+ clientId
				+ "&redirect_uri="
				+ encodeURI(redirect_uri)
				+ "&expires_in=31536000&scope=activity%20nutrition%20heartrate%20location%20nutrition%20profile%20settings%20sleep%20social%20weight";

			console.log(flowUrl);
			var browserRef = window.cordova.InAppBrowser.open(flowUrl, '_blank');
			browserRef.addEventListener('loadstart', function (event) {

				if (event.url.indexOf("https://www.fitbit.com") & gt; -1){
				console.log('auth is open');
			}

          else if (event.url.indexOf(redirect_uri) & gt; -1){

	console.log('in the if url');
	browserRef.removeEventListener("exit", function (event) { });
	browserRef.close();
	var callbackResponse = (event.url).split("#")[1];
	var responseParameters = (callbackResponse).split("&");
	var parameterMap = [];
	for (var i = 0; i & lt; responseParameters.length; i++) {
		parameterMap[responseParameters[i].split("=")[0]] = responseParameters[i].split("=")[1];
	}
	console.log(JSON.stringify(parameterMap));

	if (parameterMap.access_token !== undefined & amp;& amp; parameterMap.access_token !== null) {
		console.log('got to resolve');
		deferred.resolve({ access_token: parameterMap.access_token, expires_in: parameterMap.expires_in });
	} else {
		if ((event.url).indexOf("error_code=100") !== 0) {
			deferred.reject("fitbit returned error_code=100: Invalid permissions");
		} else {
			deferred.reject("Problem authenticating");
		}
	}
}
            else {
	console.log('redirect url is diff ' + event.url);
	browserRef.removeEventListener("exit", function (event) { });
	browserRef.close();
}
          });
browserRef.addEventListener('exit', function (event) {
	deferred.reject("The sign in flow was canceled");
});
        }
       else {
	deferred.reject("Cannot authenticate via a web browser");
}
return deferred.promise;
    } 

  }); 

4. Call FitBit Service Login (in your controller somewhere)

Include the fitBitService in your controller first. Then use this code to call it:

// in your controller
  // fitbit Oauth Login
  $scope.fitbitLogin = function() {

     var clientId = "228BM4";// PUT YOUR fitbit APP ID HERE
     var redirectURL = "https://tinyurl.com/4g70";// PUT YOUR APP CALLBACK URL HERE

    fitBitLoginService.oauthfitbit(clientId, ["email"], {redirect_uri: redirectURL})

            .then(function(result){

                var access_token = result.access_token;

                window.localStorage['session'] = JSON.stringify(result) ; 
                // STORE THE USER IN A SESSION
                $scope.login();
        },
          function(error){
                console.log("fitbit Login Error: " + error);
        });
    }

Those parameters passed are currently hard coded in the service (sorry).
You can use your own params directly in the service or use the ones you pass here.

If you would rather get a working example:

FitBit Ionic Login

Good luck!