8.4.1Provide general utility functions and JavaScript fixes in library files

We add three library files to the lib folder:

8.4.2Create a start page

The start page index.html takes care of loading CSS page styling files with the help of the following two link elements:

Then it loads the following JavaScript files:

8.5.1Summary

These steps are discussed in more detail in the following sections.

8.5.2Code the model class as a constructor function

The class Book is coded as a corresponding constructor function with the same name Book such that all its (non-derived) properties are supplied with values from corresponding key-value slots of a slots parameter.

In the constructor body, we first assign default values to the class properties. These values will be used when the constructor is invoked as a default constructor (without arguments), or when it is invoked with only some arguments. It is helpful to indicate the range of a property in a comment. This requires to map the platform-independent datatypes of the information design model to the corresponding implicit JavaScript datatypes according to the following table.

Table 8.1 Datatype mapping

Since the setters may throw constraint violation errors, the constructor function, and any setter, should be called in a try-catch block where the catch clause takes care of processing errors (at least logging suitable error messages).

As in the minimal app, we add a class-level property Book. instances representing the collection of all Book instances managed by the app in the form of an entity table:

8.5.3Code the property checks

Code the property check functions in the form of class-level (’static’) methods. In JavaScript, this means to define them as method slots of the constructor, as in Book. checkIsbn (recall that a constructor is a JS object, since in JavaScript, functions are objects, and as an object, it can have slots).

Take care that all constraints of a property as specified in the class model are properly coded in its check function. This concerns, in particular, the mandatory value and uniqueness constraints implied by the standard identifier declaration (with {id}), and the mandatory value constraints for all properties with multiplicity 1, which is the default when no multiplicity is shown. If any constraint is violated, an error object instantiating one of the error classes listed above in Section 4.1 and defined in the file errorTypes.js is returned.

For instance, for the checkIsbn operation we obtain the following code:

Notice that, since isbn is the standard identifier attribute of Book, we only check the syntactic constraints in checkIsbn, but we check the mandatory value and uniqueness constraints in checkIsbnAsId, which itself first invokes checkIsbn:

We assume that all check functions and setters can deal both with proper data values (that are of the attribute’s range type) and also with string values that are supposed to represent proper data values, but have not yet been converted to the attribute’s range type. We take this approach for avoiding datatype conversions in the user interface (“view”) code. Notice that all data entered by a user in an HTML form field is of type String and must be converted (or de-serialized) before its validity can be checked and it can be assigned to the corresponding property. It is preferable to perform these type conversions in the model code, and not in the user interface code..

For instance, in our example app, we have the integer-valued attribute year. When the user has entered a value for this attribute in a corresponding form field, in the Create or Update user interface, the form field holds a string value. This value is passed to the Book. add or Book.update method, which invokes the setYear and checkYear methods. Only after being validated, this string value is converted to an integer and assigned to the year attribute.

8.5.4Code the property setters

Code the setter operations as (instance-level) methods. In the setter, the corresponding check function is invoked and the property is only set, if the check does not detect any constraint violation. Otherwise, the constraint violation error object returned by the check function is thrown. For instance, the setIsbn operation is coded in the following way:

There are similar setters for the other properties (title, year and edition).

8.5.5Add a serialization function

It is helpful to have an object serialization function tailored to the structure of an object (as defined by its class) such that the result of serializing an object is a human-readable string representation of the object showing all relevant information items of it. By convention, these functions are called toString(). In the case of the Book class, we use the following code:

8.5.6Data management operations

In addition to defining the model class in the form of a constructor function with property definitions, checks and setters, as well as a toString() serialization function, we also need to define the following data management operations as class-level methods of the model class:

All of these methods essentially have the same code as in our minimal app discussed in Part 1, except that now

Notice that for the change operations add (create) and update, we need to implement an all-or-nothing policy: whenever there is a constraint violation for a property, no new object must be created and no (partial) update of the affected object must be performed.

When a constraint violation is detected in one of the setters called when new Book(…) is invoked in Book.add, the object creation attempt fails, and instead a constraint violation error message is created. Otherwise, the new book object is added to Book.instances and a status message is created, as shown in the following program listing:

When an object of a model class is to be updated, we first create a clone of it for being able to restore it if the update attempt fails. In the object update attempt, we only assign those properties of the object the value of which has changed, and we report this in a status log.

Normally, all properties defined by a model class, except the standard identifier attribute, can be updated. It is, however, possible to also allow updating the standard identifier attribute. This requires special care for making sure that all references to the given object via its old standard identifier are updated as well.

When a constraint violation is detected in one of the setters invoked in Book. update, the object update attempt fails, and instead the error message of the constraint violation object thrown by the setter and caught in the update method is shown, and the previous state of the object is restored. Otherwise, a status message is created, as shown in the following program listing:

Notice that optional properties, like edition, need to be treated in a special way. If the user doesn’t enter any value for them in a Create or Update user interface, the form field’s value is the empty string "". In the case of an optional property, this means that the property is not assigned a value in the add use case, or that it is unset if it has had a value in the update use case. This is different from the case of a mandatory property, where the empty string value obtained from an empty form field may or may not be an admissible value.

If there is a constraint violation exception, an error message is written to the log and the object concerned is reset to its previous state:

8.6.1The data management UI pages

Each data management UI page loads the same basic CSS and JavaScript files like the start page index.html discussed above. In addition, it loads a use-case-specific view code file src/v/useCase.js and then sets the setupUserInterface procedure of the use-case as an event handler for the page load event, which takes care of initializing the use case when the UI page has been loaded.

8.6.2Initialize the app

For initializing the app, its namespace and MVC sub-namespaces have to be defined. For our example app, the main namespace is defined to be pl, standing for “Public Library”, with the three sub-namespaces m, v and c being initially empty objects:

We put this code in the file initialize. js in the c folder.

8.6.3Set up the user interface

For setting up the user interfaces of the data management use cases, we have to distinguish the case of “Retrieve/List All” from the other ones (Create, Update, Delete). While the latter ones require using an HTML form and attaching event handlers to form controls, in the case of “Retrieve/List All” we only have to render a table displaying all books, as in the case of the Minimal App115 discussed in Chapter 3.

For the Create, Update and Delete use cases, we need to add event listeners for:

For the use case Create, we obtain the following code (in v/ createBook.js)::

Notice that for each input field we add a listener for input events, such that on any user input a validation check is performed because input events are created by user input actions such as typing. We use the predefined function setCustomValidity from the HTML5 form validation API for having our property check functions invoked on the current value of the form field and returning an error message in the case of a constraint violation. So, whenever the string represented by the expression Book. checkIsbn( formEl.isbn.value).message is empty, everything is fine. Otherwise, if it represents an error message, the browser indicates the constraint violation to the user by rendering a red outline for the form field concerned (due to our CSS rule for the :invalid pseudo class).

In addition to the event handlers for responsive constraint validation, we need two more event handlers:

While the validation on user input enhances the usability of the UI by providing immediate feedback to the user, validation on form data submission is even more important for catching invalid data. Therefore, the event handler handleSaveButton ClickEvent() performs the property checks again with the help of setCustomValidity, as shown in the following program listing:

By invoking checkValidity() on the form element, we make sure that the form data is only saved (by Book. add), if there is no constraint violation. After this handleSaveButtonClickEvent handler has been executed on an invalid form, the browser takes control and tests if the predefined property validity has an error flag for any form field. In our approach, since we use setCustomValidity, the validity.customError would be true. If this is the case, the custom constraint violation message will be displayed (in a bubble) and the submit event will be suppressed.

In the UI of the use case Update,which is handled in v/ updateBook.js,we do not have an input, but rather an output field for the standard identifier attribute isbn, since it is not supposed to be modifiable. Consequently, we don’t need to validate any user input for it. However, we need to set up a selection list (in the form of an HTML select element) allowing the user to select a learning unit in the first step, before its data can be modified. This requires to add a change event listener on the select element such that the fields of the UI can be filled with the data of the selected object.

There is no need to set up responsive validation for the standard identifier attribute isbn, but for all other form fields, as shown above for the Create use case.

The logic of the setupUserInterface method for the Delete use case is similar. We only need to take care that the object to be deleted can be selected by providing a selection list, like in the Update use case. No validation is needed for the Delete use case.

8.8.1Adding an object-level validation function

When object-level validation (across two or more properties) is required for a model class, we can add a custom validation function validate to it, such that object-level validation can be performed before save by invoking validate on the object concerned. For instance, for expressing the constraint defined in the class model shown in Figure 7.1, we define the following validation function:

When a validate function has been defined for a model class, it can be invoked in the create and update methods. For instance,

8.8.2Using implicit JS setters

Since ES5, JavaScript has its own form of setters, which are implicit and allow having the same semantics as explicit setter methods, but with the simple syntax of direct access. In addition to having the advantage of a simpler syntax, implicit JS setters are also safer than explicit setters because they decrease the likelihood of a programmer circumventing a setter by using a direct property assignment when instead a setter should be used. In other OOP languages, like Java, this is prevented by declaring properties to be ’private’. But JavaScript does not have this option.

The following code defines implicit setter and getter methods for the property

Notice that, also in the constructor definition, the internal property _title, used for storing the property value, is not used for setting/getting it, but rather the virtual property title:

We will start using implicit setter and getter methods, along with ES6 class definitions, in Chapter 11.

8.9.1Boilerplate code

An issue with the do-it-yourself code of this example app is the boilerplate code needed

While it is good to write this code a few times for learning app development, you don’t want to write it again and again later when you work on real projects. In Volume 2, we present an approach how to put these methods in a generic form in a meta-class, such that they can be reused in all model classes of an app.

8.9.2Configuring the UI for preventing invalid user input

Some of the new HTML5 input field types (like number, date or color) are intended to allow web browsers rendering corresponding input elements in the form of UI widgets (like date or color pickers) that limit the user’s input options such that only valid input is possible. In terms of usability, it’s preferable to prevent users from entering invalid data instead of allowing to enter it and only then checking its validity and reporting errors. Unfortunately, and this is quite disappointing, many browsers do, in 2017, still not have reasonable widget implementations for these HTML5 input field types yet.

8.10.1Project 1 – Validate movie data

The purpose of the app to be built is managing information about movies. Like in the book data management app discussed in the tutorial, you can make the simplifying assumption that all the data can be kept in main memory. Persistent data storage is implemented with JavaScript’s Local Storage API.

The app deals with just one object type: Movie, as depicted in the following class diagram.

In this model, the following constraints have been expressed:

Notice that the releaseDate attribute is not mandatory, but optional, as indicated by its multiplicity expression [0..1].

In addition to the constraints described in this list, there are the implicit range constraints defined by assigning the datatype PositiveInteger to movieId, NonEmptyString to title, and Date to releaseDate. In our plain JavaScript approach, all these property constraints are coded in the model class within property-specific check functions.

You can use the following sample data for testing your app:

Table 8.2 Sample data about movies

More movie data can be found on the IMDb website122.

8.10.2Project 2 – Validate country data

The purpose of the app to be built is managing information about countries. The app deals with just one object type: Country, as depicted in the following class diagram.

In this model, the following constraints have been expressed:

Notice that the militaryExpenditure attribute is not mandatory, but optional, as indicated by its multiplicity expression [0..1].

In addition to the constraints described in this list, there are the implicit range constraints defined by assigning the datatypes NonEmptyString to name, PositiveInteger to population, PositiveDecimal to lifeExpectancy and Percentage to militaryExpenditure (hint: a percentage value is a decimal number between 0 and 100). In our plain JavaScript approach, all these property constraints are coded in the model class within property-specific check functions.

You can use the sample data shown in the following table for testing your app.

Table 8.3 Sample data about countries

More data about countries can be found in the CIA World Factbook123.

8.11.1Question 1: Validation in setter (1)

Complete the following setter code fragment:

8.11.2Question 2: Validation in setter (2)

Complete the following setter code fragment:

8.11.3Question 3: Methods to add in JS class model

Consider the simple information design model shown in the class diagram.

Which of the following methods have to be added to the Publisher class in a corresponding JavaScript class model? Select one or many:

Notice that an underlined method name denotes a class-level (“static”) method.

8.11.4Question 4: Implementing constraints

Consider the constraints specified for the object type Person in the class diagram.

Which of the following JavaScript code fragments implements these constraints?

Select one: