JavaScript

Using Node.js and Charles Proxy to Mock a Server API

While working on an iPhone project for a client, I came across a situation where I needed to test some error conditions returned from a server API which I have no control over.  I ended up using Charles Proxy to remote map the server API calls to a locally running Node.js server.  Of course the proper way to do this is to write functional tests and mock the networking classes to return the appropriate responses, but that is a blog post for another day.

In this post, I’ll walk through the steps to create a simple iPhone app that interacts with a public API, using Node and Charles to simulate an error response.  This post assumes some experience writing iOS code, but all the source code will be available at https://github.com/stevenpsmith/charles-node-blog.

Create an iOS App

For the iOS app, I’ll write a simple weather app that uses the OpenWeatherMap API.  We all know the world needs yet another weather app :).

Let’s start by creating a new single view project in Xcode.  In my iOS projects I almost always use CocoaPods for dependency management.  CocoaPods makes it easy to add/remove 3rd party libraries to your iOS project.  In this app, we’ll use AFNetworking to access the API, and I’ll add MBProgressHUD in case the response time is slow and the user needs to wait for UI updates. The following assumes that you already have CocoaPods installed.

After creating the Xcode project, quit Xcode (or close the project) and create a text file named “Podfile” at the root of the project.  The contents should include the following:

platform :ios, '7.0'

pod 'AFNetworking', '~> 2.0.3'
pod 'MBProgressHUD', '~> 0.8'

With that file in place, drop to a command line and execute pod install

That will create a workspace and configure the libraries. From this point forward, we use the workspace instead of the project. The final version of the application will look like this:

Screenshot_1_14_14__12_03_PM

The code base can be found at https://github.com/stevenpsmith/charles-node-blog, but the key portion is the API call, which is below (without all the details)

[[CSMServerAPI sharedInstance] weatherForLongitude:-75.1914 
                                       andLatitude:40.1386 
    successBlock:^(NSDictionary *weatherDict) {
        // code to map API response to UI elements here
        //...
    } failureBlock:^(NSError *error) {
        //access error message parts from response here
        //....
        NSString *msg = ...
        UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"Houston - We have a Problem" 
                                                        message:msg 
                                                       delegate:nil 
                                              cancelButtonTitle:@"OK" 
                                              otherButtonTitles:nil];
        [alert show];
    }];

If everything is successful, the UI gets populated with the weather data. But, if there is a failure, we throw an alert up to the user containing the error messages (I know, alerts are not user friendly, but this is an example).

Mocking an error with Node

Now that we have the API calls in place, we need to set up our node server to fake an error response. Sticking with the theme of keeping things simple, I used the express framework for node. With node installed, I created a directory for my server app and added a ‘package.json’ file inside. The ‘package.json’ looks like the following:

{
  "name": "mock-api",
  "description": "for use with charles proxy to mock error conditions for OpenWeatherMap API",
  "version": "0.0.1",
  "dependencies": {
    "express": "~3.4.7"
  }
}

Dropping to a command line and executing npm install will set up the node server with the express module. Now we create an ‘app.js’ file and add our mock API, as follows

var express = require('express');
var app = express();

var errorJson = {error:"API Error",error_msg:"You have encountered an error thanks to Charles and Node....have a nice day!"};

app.get('/weather', function(req, res) {
  	res.json(400, errorJson);
});

app.listen(process.env.PORT || 4730);

Since our call to the OpenWeatherMap API is a get request to the /weather URI, we simply handle that request route in our code and return the response we want, which is an error containing a JSON error message. To start the server, execute node app.js at the command line and the server will be listening on port 4730.

Configure Charles Proxy for Remote Mapping

If you code against server APIs and don’t use Charles Proxy, or something like it, you really should start. It is immensely valuable for troubleshooting API calls, and the more I use it, the more nice features I find. I’ve used it for bandwidth throttling, request/response inspection, breakpoints (to change requests or responses), and now URL mapping. In order to map the OpenWeatherMap API to our local node server, access the Map Remote settings via the Tools -> Map Remote... menu. Once there, add a new remote mapping, filling out the fields as shown below. The configuration was also exported and is available at https://github.com/stevenpsmith/charles-node-blog.

Edit_Mapping_and_Map_Remote_Settings_and_Charles_3_8_3_-_Session_1

With that in place, enable remote mapping, run the iPhone app and get the following error:

Screenshot_1_14_14__11_51_AM

If you used Charles to record your network requests, you can even see the mapping occurred in the request overview notes. That’s all there is too it. You can certainly do a lot more than just simulate errors with this set up. I’ve used it to simulate server application errors as well, where server returns an http 200, but the response includes some application specific errors. Timeouts can be tested as well using the connect-timeout module. There is an example in the source code for that too.

So this is one way to intercept your API calls and modify them for testing. This is certainly no replacement for good functional tests, but simply another thing to add to your developer toolbox.

Advertisements

Using JQuery Mobile, Backbone.js for Handling Forms

Introduction

In this post, I continue the development of my basic “exercise app” that I started (and enhanced) in these posts:

Let’s add the ability to create and edit exercise records.

Adding a new activity

Just like in the earlier post, the first thing we need is a jQueryMobile page to hold the form content. Add the following to index.html:

    <div data-role="page" id="activity-form" data-add-back-btn="true">
      <div data-role="header">
        <a href="#" data-role="button" data-icon="check" data-theme="b" data-rel="back" id="save-activity-button" class="ui-btn-right">Save</a>
        <h1>New Activity</h1>
      </div>
      <div data-role="content" id="activity-form-content">
          <!-- the contents of the form will be rendered via the backbone view -->
          <form id="activity-form-form" action="#"></form>
      </div>
    </div>

In the code above, we see the typical jQueryMobile page. The main things of interest here are with respect to the anchor tag in the header div:

  • Specify data-rel=”back” to navigate to the previous page once the save is complete. This way whether we enter the form page from “Add” or “Edit”, navigation will be handled.
  • The class=”ui-btn-right” moves the button to the right side of the header bar.
  • The data-theme=”b” causes the save button to use the blue theme.

The next step is to define the template that the view will use to render the content for this page. Lets start with a simple template with an input for each attribute of an activity. To vary the UI slightly, we can use a drop down for the activity type. The basic template will look like this:

    <script type="text/template" id="activity-form-template">
        <label for="date" class="select">Date</label>
        <input type="date" name="date" id="date" placeholder="Date" value="<%= date %>" />
        
        <label for="type" class="select">Type</label>
        <select name="type" id="type">
            <option>Run</option>
            <option>Bike</option>
            <option>Swim</option>
            <option>Walk</option>
        </select>
        
        <label for="distance">Distance</label>
        <input type="text" name="distance" id="distance" placeholder="" value="<%= distance %>" />
    
        <label for="minutes">Minutes</label>
        <input type="tel" name="minutes" id="minutes" placeholder="" value="<%= minutes %>" />
        
        <div data-role="fieldcontain">
            <textarea name="comments" id="comments" placeholder="Comments"><%= comments %></textarea>
        </div>
    </script>

Notice that we are using the HTML 5 input types for some of the fields. Browser support for these will vary, but worst case they will deprecate to a text input type. For example, on iOS, the date will present a native date picker, whereas on Android (or Chrome, etc.) the field will behave as a standard text input.

Now we need to navigate to the form page. This requires modifying the “Add” button on the list page.

    <div data-role="page" id="activities">
      <div data-role="header">
        <h1>Activities</h1>
        <a href="#activity-form" data-role="button" data-icon="add" id="add-button" class="ui-btn-right">Add</a>
      </div>
      <div data-role="content">
          <!-- the contents of the list view will be rendered via the backbone view -->
      </div>
    </div>

In order to transition to the form page, the href attribute on the anchor tag (i.e. button) needs to be changed to the id of the form page, as shown in line 4 above. The final step in presenting the form page to support adding exercise activities is modifying the add button click handler.

    $('#add-button').live('click', function(){
        var activity = new exercise.Activity(),
            activityForm = $('#activity-form-form'),
            activityFormView;

        activityFormView = new exercise.ActivityFormView({model: activity, viewContainer: activityForm});
        activityFormView.render();
    });

Here we create an empty model, grab the form node, create a new exercise.ActivityFormView with these objects, and render the form. Testing at this point reveals a problem. The screen shot below doesn’t look right 😉

Investigating the browsers console reveals an error: Uncaught ReferenceError: distance is not defined. This is due to the underscore template trying to access an undefined attribute of the model. The issue is described here with some work arounds. I found the easiest workaround is to provide defaults for the model, as shown below.

     exercise.Activity = Backbone.Model.extend({
        defaults: {
            date: '',
            type: '',
            distance: '',
            comments: '',
            minutes: ''
        }
     });

Now the form renders properly. With the help of some CSS, we can tighten things up a little and end up with the version on the right (the CSS is part of the source code, see the links at the end of this post).

Editing an existing activity

With the template and the view already implemented, adding edit functionality is pretty straightforward. The first step is to update the href attribute of the edit button on the activity-details page.

    <div data-role="page" id="activity-details" data-add-back-btn="true">
      <div data-role="header">
        <a href="#activity-form" data-role="button" data-icon="arrow-d" id="edit-activity-button" class="ui-btn-right">Edit</a>
        <h1>Activity Details</h1>
      </div>
      <div data-role="content" id="activity-details-content">
          <!-- the contents of the list view will be rendered via the backbone view -->
      </div>
    </div>

As we did with the add button, this should reference the activity-form page. Now we need to add the click event handler for the edit button.

    $('#edit-activity-button').live('click', function() {
        var activityId = $('#activity-details').jqmData('activityId'),
            activityModel = exercise.activities.get(activityId),
            activityForm = $('#activity-form-form'),
            activityFormView;
        
        activityFormView = new exercise.ActivityFormView({model: activityModel, viewContainer: activityForm});
        activityFormView.render();
    });

If you recall, the activityId is passed to the details page view the list item click event handler. We reuse that fact here to retrieve the activity from the collection.

Time to test. Selecting an activity renders the detail page. Clicking the edit button renders the form, pre-filled with the activity details. Everything looks great.

But wait…..that screenshot was from the browser on which I am testing. We should always test on devices as well. Earlier I mentioned that we are using some of the HTML 5 input types (i.e. date). The nice thing is that iOS 5 will render a nice date picker for date types. The challenge is that the date needs to be in a specific format for that to work. The image below shows what the form page looks like on an iOS device.

Fixing the Date

No date value is showing up on our iOS device. This is a known issue. So, for iOS we need the date format as yyyy-mm-dd, while for other platforms, I prefer the date to be shown as mm/dd/yyyy. I do realize that date formats should be localized, but for the purpose of this example I would like to keep it straightforward and specify mm/dd/yyyy as the display date format. There are many options that could be pursued here, but to demonstrate some more capabilities of Backbone.js, we will modify our model to help meet our date requirements.

First, the date attribute in our JSON feed is a string. Lets convert this to a date when the data is retrieved from the server, then when we need the date, we can just manipulate it as needed. This can be accomplished several ways.

  • Implement a parse method on the collection that converts the String to a Date as the data is being fetched from the server.
  • Implement a method on the model that converts the date String to a Date and sets it on the model. This would then require the calling code to know to call this method depending upon the circumstance.
  • Override the set method on the model, look for the date attribute, then convert it to a date as needed.

The last option is the most seamless approach. Add the following method to the Activity model.

        set: function(attributes, options) {
            var aDate;
            if (attributes.date){
                //TODO future version - make sure date is valid format during input
                aDate = new Date(attributes.date);
                if ( Object.prototype.toString.call(aDate) === "[object Date]" && !isNaN(aDate.getTime()) ){
                    attributes.date = aDate;
                }
            }
            Backbone.Model.prototype.set.call(this, attributes, options);
        }

Also, update the defaults so that the date attribute is now a Date instead of a String.

        defaults: {
            date: new Date(),
            type: '',
            distance: '',
            comments: '',
            minutes: ''
        },

While this clearly isn’t the most robust date handling, it is fine for now. In a future version, this should be improved.

Next, add attributes to our model that will format the date in the ways we need it (i.e. mm/dd/yyyy and yyyy-mm-dd).

        dateInputType: function(){
            return exercise.formatDate(this.get('date'), "yyyy-mm-dd"); //https://github.com/jquery/jquery-mobile/issues/2755
        },
        
        displayDate: function(){
            return exercise.formatDate(this.get('date'), "mm/dd/yyyy");
        }

The formatDate function is a simple date formatter to meet our specific needs and can be found in the source code that accompanied this post (see link at the bottom of the post).

Now, how do we use these new methods in our view. The first thing to realize is that we pass the template JSON. The default implementation of the toJSON method of a Backbone.js model will not include these functions. Therefore, we need to override the toJSON method.

        toJSON: function(){
            var json = Backbone.Model.prototype.toJSON.call(this);
            return _.extend(json, {dateInputType : this.dateInputType(), displayDate: this.displayDate()});
        }

Here, we are using the Underscore.js extend to add our attributes to the standard Backbone JSON. Now, we need to modify our view templates to use the appropriate JSON attributes.

    <script type="text/template" id="activity-list-item-template">    
      <li><a href="#activity-details" identifier="<%= id %>"><%= displayDate %> - <%= type %></a></li>
    </script>
    
    <script type="text/template" id="activity-details-template">
        <h3><%= type %></h3>
        <ul data-role="listview" id="activitiy-fields" data-inset="true">
          <li>Date: <%= displayDate %></li>
          <li>Minutes: <%= minutes %></li>
          <li>Distance: <%= distance %></li>
          <li>Comments: <%= comments %></li>
        </ul>
    </script>
    
    <script type="text/template" id="activity-form-template">
        <label for="date" class="select">Date</label>
        <% if (navigator.userAgent.indexOf('iPhone') >= 0 || navigator.userAgent.indexOf('iPad') >= 0) { %>
            <input type="date" name="date" id="date" placeholder="Date" value="<%= dateInputType %>" />
        <% }else{ %>
            <input type="date" name="date" id="date" placeholder="Date" value="<%= displayDate %>" />
        <% } %>
        
        <label for="type" class="select">Type</label>
        <select name="type" id="type">
            <option>Run</option>
            <option>Bike</option>
            <option>Swim</option>
            <option>Walk</option>
        </select>
        
        <label for="distance">Distance</label>
        <input type="text" name="distance" id="distance" placeholder="" value="<%= distance %>" />
    
        <label for="minutes">Minutes</label>
        <input type="tel" name="minutes" id="minutes" placeholder="" value="<%= minutes %>" />
        
        <div data-role="fieldcontain">
            <textarea name="comments" id="comments" placeholder="Comments"><%= comments %></textarea>
        </div>
    </script>

Notice that lines 16-21 include some conditional logic. This is a very basic device detection check to determine which date format to use. There are plug-ins,etc. that provide more robust alternatives, but to keep things clear, this will serve our needs. The screen shots below show the desktop browser and the iOS versions of the form with the appropriate date handling.

Saving

The final step is to implement the save functionality.

    $('#save-activity-button').live('click', function(){
        var activityId = $('#activity-details').jqmData('activityId'),
            activity,
            dateComponents,
            formJSON = $('#activity-form-form').formParams();
        
        //if we are on iOS and we have a date...convert it from yyyy-mm-dd back to mm/dd/yyyy
        //TODO future version - for non-iOS, we would need to validate the date is in the expected format (mm/dd/yyyy)
        if (formJSON.date && ((navigator.userAgent.indexOf('iPhone') >= 0 || navigator.userAgent.indexOf('iPad') >= 0)) ){
            dateComponents = formJSON.date.split("-");
            formJSON.date = dateComponents[1] + "/" + dateComponents[2] + "/" + dateComponents[0];
        }
        
        if (activityId){
            //editing
            activity = exercise.activities.get(activityId);
            activity.set(formJSON); //not calling save since we have no REST backend...save in memory
        }else{
            //new (since we have no REST backend, create a new model and add to collection to prevent Backbone making REST calls)
            activity = new exercise.Activity(formJSON);
            activity.set({'id': new Date().getTime()});  //create some identifier
            exercise.activities.add(activity);
        }
    });

I used the JavascriptMVC formParams jQuery plugin (found here) to convert my HTML form into a JSON object. Then, the date is converted into the appropriate format. One thing to note here is that since we have no real back end server for Backbone to interface with, we don’t call save on the model or create on the collection. These methods would cause the appropriate REST calls to the server.

Since the click event on the list view passes the activityId to the details page, we can use this to determine if we are adding or editing. One thing to consider here is that the user may click an existing activity, then go back to the list, then click add. In this case, the activityId from the previously selected activity is still attached to the activity-details view. This will cause our save implementation to think we are editing. To prevent this, we should remove the activityId from the activity-details page when adding a new activity. This can be accomplished by adding the highlighted line below to the add handler.

    $('#add-button').live('click', function(){
        var activity = new exercise.Activity(),
            activityForm = $('#activity-form-form'),
            activityFormView;
    
        //clear any existing id attribute from the form page
        $('#activity-details').jqmRemoveData('activityId');
        activityFormView = new exercise.ActivityFormView({model: activity, viewContainer: activityForm});
        activityFormView.render();
    });

The final step is to make sure our views are updated accordingly when we add or modify activities. Since the details view retrieves the activity before the page loads, no change is required here. But our list view only renders during the initial page load. This is where we can take advantage of Backbone event binding. A few small additions to the ActivityListView and things will be handled.

        initialize: function() {
            this.collection.bind('add', this.render, this);
            this.collection.bind('change', this.changeItem, this);
            this.collection.bind('reset', this.render, this);
            this.template = _.template($('#activity-list-item-template').html());
        },

        ...

        changeItem: function(item){
            this.collection.sort();
        }

The change event will bubble up from the model and fires as a result of the set being called. In the changeItem method, we sort the collection to handle any changes in the date attribute. The sort method call will cause the reset event to fire, which then results in the render method invocation, causing the list to be re-rendered. This keeps everything in the correct order. The add event will be fired when we add a new activity to the collection. Since we implemented the comparator method on the collection, models added to the collection will be added in the appropriate order.

We can now add and edit activities. Start exercising 🙂

The source code for this post can be found here

From a List to a Detail View using jQuery Mobile and Backbone.js

In my previous post I built a basic application to demonstrate the use of Backbone.js with jQueryMobile (JQM). The introduction can be found here, with a brief subsequent post on sorting collections here. In this post, I would like to add the capability to view the details of the items presented in the list view.

The first step is to create a new JQM page to display the details view. JQM makes it pretty easy to add pages to your application. I added the following code to the index.html file:

<div data-role="page" id="activity-details" data-add-back-btn="true">
  <div data-role="header">
    <a href="#" data-role="button" data-icon="arrow-d" id="edit-activity-button" class="ui-btn-right">Edit</a>
    <h1>Activity Details</h1>
  </div>
  <div data-role="content" id="activity-details-content">
      <!-- the contents of the list view will be rendered via the backbone view -->
  </div>
</div>

This will create the structure for the page, and Backbone.js will be used to fill in the content div based on the record tapped (or clicked) within the list view. The next step is to define the template for the details view. This is the pattern that I follow when developing using jQueryMobile and Backbone.js. The template can be inserted below the list item template in index.html.

    <script type="text/template" id="activity-details-template">    
        <h3><%= type %></h3>
        <ul data-role="listview" id="activitiy-fields" data-inset="true">
          <li>Date: <%= date %></li>
          <li>Minutes: <%= minutes %></li>
          <li>Distance: <%= distance %></li>
          <li>Comments: <%= comments %></li>
        </ul>
    </script>

I decided to embed the details in a read-only list view, this way jQueryMobile will provide some reasonable styling. Since the point here is to demonstrate Backbone and jQueryMobile playing together, I didn’t want to have to spend much time on style 🙂

Next, we need to define the Backbone view that will use the template to render the appropriate content. All this View needs to do is apply the model to the template and append it to the HTML container defined when the View is instantiated.

    exercise.ActivityDetailsView = Backbone.View.extend({
        //since this template will render inside a div, we don't need to specify a tagname
        initialize: function() {
            this.template = _.template($('#activity-details-template').html());
        },
        
        render: function() {
            var container = this.options.viewContainer,
                activity = this.model,
                renderedContent = this.template(this.model.toJSON());
                
            container.html(renderedContent);
            container.trigger('create');
            return this;
        }
    });

In order to retrieve the correct model to bind to the details view, we need to know what row in the list view was clicked (or tapped). To do this, we can bind to the click event of the item in the list view. This can be accomplished by modifying the ActivityListView render method. Here is the current version of the list view:

    exercise.ActivityListView = Backbone.View.extend({
        tagName: 'ul',
        id: 'activities-list',
        attributes: {"data-role": 'listview'},
        
        initialize: function() {
            this.collection.bind('add', this.add, this);
            this.template = _.template($('#activity-list-item-template').html());
        },
        
        render: function() {
            var container = this.options.viewContainer,
                activities = this.collection,
                template = this.template,
                listView = $(this.el);
                
            $(this.el).empty();
            activities.each(function(activity){
                listView.append(template(activity.toJSON()));
            });
            container.html($(this.el));
            container.trigger('create');
            return this;
        },
        
        add: function(item) {
            var activitiesList = $('#activities-list'),
                template = this.template;
                
            activitiesList.append(template(item.toJSON()));
            activitiesList.listview('refresh');
        }
    });

The key area to focus on here is lines 18-20, where the activity item HTML is rendered and appended to the list view. This is where the modification needs to occur. Each activity item HTML element needs to be bound to a click event. In this click event, the activity id will need to somehow be passed to the details view so the appropriate look up can occur. There are several ways to do this. Approaches I have used in the past include the use of jQuery.jqmData(...) or session local storage (assuming HTML5). In this example, we will use the jqmData method. The trick is to capture the id during the rendering of the list so that it can be used during the execution of the click event. Below are the required modifications to lines 18-20 of the previous code snippet.

activities.each(function(activity){
   var renderedItem = template(activity.toJSON()),
       $renderedItem = $(renderedItem);  //convert the html into an jQuery object
   $renderedItem.jqmData('activityId', activity.get('id'));  //set the data on it for use in the click event
   $renderedItem.bind('click', function(){
         //set the activity id on the page element for use in the details pagebeforeshow event
        $('#activity-details').jqmData('activityId', $(this).jqmData('activityId'));  //'this' represents the element being clicked
   });
   listView.append($renderedItem);
});

The first thing is to capture the rendered activity item HTML in a variable and cast it to a jQuery object, as can be seen in lines 2-3. Line 4 is where the activity id data is attached to our activity item HTML element. Then, the bind event (lines 5-8) retrieves the attached data and sets it on the activity details HTML element, which is our activity details page HTML. Per the jQuery documentation, this within a bind method refers to the DOM element to which the event handler is bound. We can use that to get at the activity id data and attach it to the details view. This gives us the ability to pass the appropriate id at runtime to the details page.

Now we need to wire all this together. The typical pattern I follow is to use the jQueryMobile pagebeforeshow event to set up everything needed to render a complete page. This acts as my controller.

$('#activity-details').live('pagebeforeshow', function(){
    var activitiesDetailsContainer = $('#activity-details').find(":jqmData(role='content')"),
        activityDetailsView,
        activityId = $('#activity-details').jqmData('activityId'),
        activityModel = exercise.activities.get(activityId);
    
    activityDetailsView = new exercise.ActivityDetailsView({model: activityModel, viewContainer: activitiesDetailsContainer});
    activityDetailsView.render();
});

This code retrieves the id data attached to the activity details element, looks up the model using the Backbone API (line 5), instantiates the view with the model and the view container, and calls the render method to render the final HTML. The end result looks like this:

The source code for this post can be found here. Note that this is a branch of my repository for this sample app. Each blog post associated with this code base resides on a separate branch.

Related Posts:

This is a re-post of my original found here.

Executing JavaScript from Objective-C in an iOS App

The other day, I found the need to execute some JavaScript from a native iOS application.  After doing a little research, I discovered that the UIWebView has a stringByEvaluatingJavaScriptFromString method.  So I decided to create a quick little proof-of-concept to see how to invoke some JavaScript using this. I am not going to walk through all the details of creating a project, etc. and will cut right to the important aspects of making the JavaScript call from the objective-c code.

We are going to need a JavaScript function to call and a UIWebView to evaluate the JavaScript. It would be nice if the JavaScript function took a parameter as well, so we could also prove out the passing of values.  Here is the JavaScript function that I will use:

var area = function(diameter) {
     var radius = diameter/2,
     area = Math.PI * radius * radius;
     return Math.round(area*100)/100;
};

Next we need to load this JavaScript in a UIWebView to allow for the call to stringByEvaluatingJavaScriptFromString. This can be done several ways. One option is to embed JavaScript in a dummy html file and load that into the UIWebView. You could also load an externalized JS file into the dummy html file. I chose to load some inline html into the UIWebView that included a reference to the externalized JS file. This way I don’t need any extraneous files hanging around. Also, I wanted the JS to live in its own file and act as a library I could use in other (non-objective-c) applications, so I placed it in the calc.js file. Here is the code to load the JS file into the UIWebView:

[webView loadHTMLString:@"<script src=\"calc.js\"></script>" 
              baseURL:[NSURL fileURLWithPath:[[NSBundle mainBundle] resourcePath]]];

Here is the code to make the JS call:

NSString *function = [[NSString alloc] initWithFormat: @"area(%i)", diameter];
NSString *result = [webView stringByEvaluatingJavaScriptFromString:function];

Now, there is one important safety tip to make this actually work: The UIWebView must actually be loaded a view. It doesn’t have to be visible, but in order for the WebKit engine to execute the JS, it must be on a view.

Also, when you perform a build, you will get the following warning:

no rule to process file ‘$(PROJECT_DIR)/Classes/calc.js’ of type sourcecode.javascript for architecture i386

This can be eliminated by moving the calc.js file from the Compile Sources Task to the Copy Bundle Resources task within your project target. It should look something like the image below.

Project Pane - Target

That should do it. Once all these pieces are in place, you will have objective-c executing JavaScript.

The source code can be found on github here.

Please forgive the UI (and the lack of memory mgmt), as I just threw this together 🙂