JavaScript

History

• Originally designed as a language to run in the browser to
  • Add nice UI effects
  • Reduce the need for page reloads (e.g., by performing form validation locally).
• 1995: Netscape creates LiveScript (quickly changed to JavaScript)
• Microsoft introduces VBScript and then Jscript
• 1996: Netscape proposes ECMAScript specification as a standard
  • ECMA: European Computer Manufacturers Association
  • https://www.ecma-international.org
• 1997: First version of ECMA-262
• 2015: ES6 – later called ES2015
  • Introduced many of the “modern” features (classes, imports, () => notation, etc.)
• New (minor) version every year since https://www.w3schools.com/js/js_versions.asp
• Current version is 10th Edition (also called ES2019)
• Has capability to modify the web page (i.e., the DOM), thereby giving web apps a more native feel. (More on that later.)

Basics

• Similar to Java/C/C++
• I will focus on the key features – especially those that are different from most programming languages.
• JavaScript is Dynamically Typed (as opposed to Java/C/C++ which are statically typed)
  • What does it mean for a language to be dynamically typed?
    • Variable types aren’t declared.
  • What are the pros and cons?
    • Pro: Can leverage polymorphism without elaborate inheritance/interface structures.
    • Con: Compiler can’t identify when you’ve tried to call a method that doesn’t exist.
    • See dynamicTyping.js in the course sample code.
    • Pro: This lack of typing makes functional programming less cumbersome because we don’t have to jump through hoops to specify the type of the “lambda” parameter.
    • Con: Again, mistakes aren’t caught until runtime.

Key features / differences

• See keyFeatures.js
• Template strings
  • You can put code inside the ${}. It doesn’t have to just be a variable.
• Uses “Java-style” comments (both // and /* ... */)
• Integers and floating point are the same type. (Try typeof(1) and typeof(3.14159));
• All objects behave like associative arrays.
• == vs ===
  • == converts both parameters to the same type before comparison (called coercion)
  • === does not coerce parameters. If the parameters are different types, the result is false.
  • Use === unless you have a good reason to use ==.
  • When you use == add a comment explaining why.
  • See keyFeatures.js
• null and undefined are different.
  • null == undefined is true: Both convert to false
  • null === undefined is false: They are different types.
  • Be sure to check for both null and undefined.
• Nullish Coalescing Operator (??) https://www.w3schools.com/js/js_2019.asp#mark_nullish_coalescing
• let vs var
  • Variables declared using let have block scope (are local to the block)
  • Variables declared using var are visible throughout the function (are not local to the block.)
  • Use let unless you have a good reason to use var.
  • When you use var, and a comment explaining why.
• Some methods are asynchronous. (See asynchronous.js)
• Deconstructing arrays
• Deconstructing objects
• Use a linter (e.g., JSHint) https://jshint.com
• Modules: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Modules

Express

1. npm init (accept defaults)
2. npm install express --save
3. npm install ejs --save (skip if you prefer to use Pug)

4. Create index.js with the following content:

    const express = require('express')
    const app = express()
    const port = 3000
       
    app.get('/', (req, res) => res.send('Hello World!'))
       
    app.listen(port, () => console.log(`Example app listening on port ${port}!`))
   

5. Launch node index.js
   • visit localhost:3000
6. Create a directory named views
   • By default, templates go here.
   • Convention vs. configuration.
   • If you want to change it: app.set('views', './your_preferred_location')

7. Create a file in views named hello.ejs (or hello.pug, if you prefer)

   <p>Hello, there <%= name %></p>
   <p>Not a bad first page.</p>
   

8. Add app.get('/', (req, res) => res.render('hello.ejs', { name: "Bob" })); to index.js
   • You can also set a default template engine: app.set('view engine', 'ejs')
   • Doing so allows you to leave off the .ejs

9. Launch the server and visit the root page

10. Add a route
    • print the req object.
    • print just req.query
    • Use req.query set a variable in the template

• Show multiplication table and processing of POST

MVC (Model, View, Controller)

• This is a general software development design pattern. It’s not just for web apps
• Model
  • Represents the data / objects
  • Access the database
  • Provides the business logic
• View
• Renders the web page
• Controller:
• Ties model and view together
• Basic workflow
  1. Receives input / action (e.g., HTTP request)
  2. Calls appropriate model (runs the correct business logic)
  3. Uses the results of the model’s actions to update view accordingly.
• Look at Toy.js
  • Primarily a container for a DB record
  • class methods represent DB accesses
• The module.exports = line is a module system called CommonJS
  • CommonJS was designed by Node to prevent namespace pollution (in other words, to avoid name collisions when importing multiple modules)
  • module.exports is an object contains the exported elements of a module.
    • Sometimes it is the single exported object
      • (In the case of express, the module returns a function that creates the server object.)
    • Sometimes it is an object that maps names to the several “public” elements of a module.
  • Node provides a require method to load the code and return the exports object
    • The Toy module only returns one thing: The Toy class.
• The most recent versions of JavaScript use new import and export statements.
  • Node supports these statements on an “experimental” basis.
• The routing in index.js maps each web request (combination of method and path) to a method on a controller.
• That controller method
  1. Accesses the model
  2. Renders a view
• Step through the following
  • Index
  • Show
  • Create
  • Update
  • Point out the use of partials.
  • Point out the use of the Array “find” method.

Convention over configuration

• By convention, Express uses pug.
  • If you choose to use pug, you don’t need to type anything.
  • If you choose to go against convention (e.g., use EJS), you need to add a line of configuration.

• Similarly, by convention, the EJS templates go in a folder named views; but you can explicitly configure it.

• Notice that the set of routes in index.js follows a pattern
  • GET /modelName => modelNameController#index
  • GET /modelName/:id => modelNameController#show
  • etc.
• If we agree to use this pattern for all models, we can easily write a function to automatically generate these routes.
• We could also have this routing function automatically render a conventionally-named view.
• We could even have a program write an outline of our model, controller, and all of our views.
• Rails (and many other frameworks) do all of this.
• They have steep learning curves; but, allow you to be very expressive/efficient once you become comfortable.

Testing / Jest

• Discuss the addition of isValid() to the Model.
• Add Jest to the project https://jestjs.io
  • npm install jest --save
  • edit the “test” line of packages.json to say "test": "jest"
  • Add a file test/Toy.test.js and add some test to it.
  • run `npm run test
• Notice the use of “!this.name” to check for a missing name (including an empty string.)
• Notice that !this.price doesn’t work for checking missing values, because it will also disallow 0.0
• Explain parseFloat(this.price) != this.price
• Demonstrate .only
• Demonstrate toBeTruthy and toBeFalsy
• Be sure to watch tests fail. Watch what happens if we
  1. Make a mistake in create and
  2. Make a mistake in the test (by using a previous description).
• Review the describe/it syntax in Toy.test.js
  • Run npm run test test/Toy.test.js
  • Notice how the tests read as a sentence and describe the expected behavior (hence the describe block)
• To run tests by name
  • run npm run test -- -t '#create'
  • The -- causes the -t '#create' to be passed to jest instead of consumed by npm.
  • We need single quotes around #create because # has special meaning to the shell.

Mocking

• Look at ToyController.test.js
  • To verify the behavior or this controller, we primarily need to verify how it interacts with the request and response objects
  • Setting up these objects (and avoiding any pitfalls) would be time consuming and difficult, so we use mock objects.
  • We “mock” the response object for use in the #index method by simply creating an object with a render method.
  • This mocked render method, doesn’t actually do anything, it just keeps track of how it is used (how many times it is called, what parameters are passed, etc.)
  • This tracking is done by jest.fn
  • explain the test renders the index view
  • explain the #show tests
  • explain the #new tests
    • Explain how Toy and its constructor are mocked

Databases / Asynchronous programming / Promises

• You can use the sqlite package to access a simple, file-based SQL database.
• Challenge: DB access is asynchronous.
• Examine sqliteWithCallbacks/whyCallbacks.js
• examine SqliteToyDB.js
  • Notice that all and find need to take a callback method.
  • They should really take a second callback for errors.
• This example isn’t too bad; but, if we had a sequence of database accesses, you would need nested callbacks, which can get confusing and hard to maintain.
• Promises provide a means of organizing chains of callbacks.
  • See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Using_promises for an example of the “pyramid of doom”
  • Examine sqliteWithPromises
  • Note: At this point, the difference is small.
• Async / Await
  • We can use new keywords async and await to avoid callbacks at all.
  • If a method returns a promise, then you can just say await to instead obtain the resolve value.
  • Any method that uses await must be declared as async.
  • async methods implicitly return promises, so you may need to await an async method.
  • Show updated MVC
    • Point out that when updating, we need to use function() instead of ()=> to have access to this.