wesley-waters/uihp-challenges-april23

The Hard Parts of UI Development - Challenges

Summary

The purpose of these challenges is to gain an under-the-hood understanding of building user interfaces in the web browser. Each challenge will go hand-in-hand with a section of The Hard Parts of UI Development Course by Will Sentance from Frontend Masters.

Throughout these challenges, we will cover topics including HTML, JavaScript, & the DOM API in the web browser; one-way data binding and "state-driven" views; the virtual DOM and "composable" UI; and performance optimizations.

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Part 1: Building an Interactive UI

In UI Engineering we have 2 simple goals:

• Display content (data) for users to see.
• Enable our users to interact with the content they see, and then change it.

1. First, let's take a look at the index.html file. Open it in the browser.
   • To do this in your terminal, make sure you have navigated to the correct directory and run the command open index.html. This should automatically open the file in your default web browser.

What do you see? Your tab or window should show the text you see inside the <title> in index.html, but the page itself should be totally blank. You can also inspect the page and view your html code in Chrome DevTools. For more info about how to use Chrome DevTools, check out their docs.

2. Now, uncomment the <div> and <input> in index.html and refresh the page in the browser. (Use command + / or control + / to comment/uncomment code) When our HTML loads now, it will populate the DOM and we can see our input box on the page! Feel free to add some text to your <div> and refresh so that you can see it render to the page as well.

3. Next, type something into your input box. The pixels on the page show what you have typed - Remember that the underlying DOM data for the input node has also updated to reflect your change!

4. Uncomment the line containing the stylesheet in index.html to add some pizazz to our page - remember, when this loads, it will populate the CSSOM, similar to how our HTML populates the DOM with elements to render.

Now what? Even if we can change the data on the DOM, we can't do anything with that data or access it directly. We would still need some kind of logic to make something happen in the event that a user types in our input or a button is clicked. Our html is only being loaded once and we can't run any code in the DOM. Therefore, we need JavaScript to create, save, and update data.

5. Uncomment the <script> tag in index.html. This script acts as a link between the html and the code in the linked JavaScript file. When our script loads with our HTML in the browser, the JavaScript engine will start running and allow us to run our JS code in the browser directly (and access everything else we get with the JS runtime, including memory to store data).

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Extension Challenge: Document Object - In our JavaScript runtime, we have access to some very useful APIs, including document. In part1.js, if you console.log(document), you will see an object in the browser dev tools console. This document object also has a hidden property that acts as a link to the DOM.

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6. In part1.js, declare a variable 'post' and initialize it to a string that is your name. Congrats - we have data! Now, how can we use it to update the DOM and what we see in our view?

If you're unfamiliar with DOM manipulation, take a look at the docs on MDN, and particularly the querySelector method (a hidden property on the document object), which allows us to query the DOM to select a specific DOM element and create an object in our JavaScript memory with a hidden "link" to access it. That object will have "property-methods" to get or set the data on that DOM element.

7. Query the DOM to select the input node on the DOM and assign the resulting JS object to a variable called 'jsInput'. Now do the same for the the div node on the DOM and assign the resulting JS object to a variable called 'jsDiv'.

8. Let's use our JS data to update the DOM data. Our jsDiv object will have some "getter/setter" property-methods, including textContent. Set the jsDiv.textContent to be the value of our post variable. Refresh the page in the browser, and you should see that string as the text in the div.

9. Now let's update our JavaScript data based on our user interactions. First, edit your variable declaration for post at the top of the file to initialize it to an empty string instead.

   Declare a function 'handleInput' that will "handle" what we want to do with what the user types in the input. Use the value getter/setter method available on our jsInput object. When the user types into the input, reassign post to hold that text.

   Set the textContent on the div to to be the value of post here in handleInput instead.

10. Finally, let's create an event handler that will run our handleInput function in the event that a user types in input. There are several ways to do this, but one is the oninput property on the jsInput object. Us this to set handleInput on our input DOM element as a callback.

We should now have a full User Interface (UI) which addresses our two main goals:

• Display content (data inside computer/from internet) as the 'view' for users to see, and
• Enable the user to interact (tap, click, etc) with the 'view' they see and change it (by changing the udnerlying data & updating the view).

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Part 2: One-way Data Binding

Now that we have a simple application our users can interact with, how can we make our UI more sophisticated? One thing we can do is implement one-way data binding, a popular paradigm for tackling the essential engineering challenge of keeping the 'view' consistent with (and dependent on) our underlying data.

1. In index.html, edit the script tag to use part2.js as the src file.

2. What if we wanted to add some placeholder text to the input box? In your JS file, set the value of the input box to be the string 'What's on your mind?' and refresh the browser.

Click into the input and type something. Notice that the user needs to manually delete the placeholder text that is in the input box every time they want to type in a new input.

3. Let's add a click handler 'handleClick' for the input box which sets the value inside the input box to an empty string when the user clicks in it to type. HINT: Make sure to add handleClick as a callback to the input element.

Now we're changing our 'view' based on several different possible user interactions. How can we make these changes more predictable? Let's restrict every change to view to be via:

• an update of 'data'
• a run of a single dataToView convertor function.

4. To do this, let's create a function 'dataToView'. It should:

   • Update the value in our input box on the DOM to be whatever post currently is. HINT: post should now start off as undefined, and if it is undefined when dataToView is invoked, we should populate the value in the input box to be the string containing 'What's on your mind?'. This also means dataToView needs to be invoked when the page is first loaded.

   • Update the content of the div on the DOM to be our current data.

5. Now that the dataToView function uses our JS data to update the DOM content, we need to make sure it is invoked after our data changes. What adjustments can we make to our event handlers so that they only make changes to the underlying data, and what should happen as soon as a change is made?

6. While it may not be efficient, we can have our dataToView run so often that any change to data will instantly propagate using a setInterval function at a rate that is close to the browser refresh rate.

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Extension Challenge: Submit Button
Add a submit button that will create and save a new post. How can you implement this kind of functionality in your application? In addition, set up some logic for creating divs that hold previous 'posts' so that the user is able to see all their previous posts.

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Part 3: The Virtual DOM

In our first two UI Hard Parts challenges, we displayed content and let our users interact with it to change the underlying data, and implemented one-way data binding to ensure our view is always dependent on and consistent with that data.

In part3.js, we are describing key parts of the UI in dataToView: the contents (data) and how to display it. What if we described the elements as well? This would make our funnction a complete description of the data and view.

1. In index.html, edit the script tag to use part3.js as the src file. Then comment out or delete the input and div elements.

Instead, let's create our DOM elements with JavaScript. A UI Component is a function that fully creates elements and relates their data to the view.

2. In part3.js, uncomment the function 'component' and build it out. You will see it has already been started for you.

   • The first and last lines of code in this function are there to make sure that once your user has clicked in the input, they will stay "clicked" and able to type as component is repeatedly called (hint!) to auto-update the DOM.

   This function will combine all of our previous functionality into one:

   • Creating DOM elements (instead of selecting them).
     • Feel free to keep the jsInput and jsDiv variable declarations at the top of the file, but unitialized - you will reassign the values to be the new objects you create.
   • Setting their contents based on our JS data.
     • Once this happens inside component, we should no longer need our dataToView function.
   • Creating and attaching event handlers to them.
   • Displaying our new DOM elements in the view.

   In order to achieve this last step, we need to attach or append them to the body of the DOM. There a a couple of ways to do this, but to make sure we are replacing our nodes with an updated version when our data changes (instead of accidentally attaching multiple copies of our input or div), use replaceChildren.

Now, our view should look the same as it did before, but our code should consist of 3 parts - our declared variables at the top of the file, the component function (where all of our functionality will be now), and setInterval.

However, our code is still fairly imperative - just glancing at it, it's hard to tell what our view will look like. The more "visual" our code is (like HTML), the easier it is for us to work with as developers.

For example - imagine we wanted the text content of a div element to be the string "I live in (user's location)!". We can use the concat method to build out our string piece by piece:

let userLocation = 'LA';

let textToDisplay = 'I live in ';
textToDisplay = textToDisplay.concat(userLocation);
textToDisplay = textToDisplay.concat('!');

However, this is not very visual or declarative. So instead, we can use a template literal with string interpolation:

textToDisplay = `I live in ${userLocation}!`;

As we can see, this mirrors what our visual/graphic output will be. Could we do something similar with our main code creating visual elements?

Let's start with a "unit of code" representing each piece of our view.

const divInfo = ['div', `Hi, ${myName}!`];

In this example, our divInfo is an array with the details of a DOM element - just by looking at our code, we can tell what it will look like (the type (div) and the text that will display inside).

3. In part3.js, write a function 'convert' that will take in a node (an array of details like our 'divInfo' example above). This function should use that array to create a new DOM element and set its content, and return its linked JavaScript object.

   With this function, we can now produce DOM elements from any list of info that visually mirrors what we will see in our view.

Next, let's create our virtual DOM - blocks of code (or a list) representing each piece of our view.

4. Declare a global variable 'vDOM' but do not initialize it.

5. Write a function 'createVDOM' that returns a list (array) containing the following two sub-arrays:

[
  'input',
  myName,
  function handle() {
    myName = jsInput.value;
  },
][('div', `Hello, ${myName}!`)];

• Notice that in each sub-array, index [0] is the type of DOM element we want to create, index [1] has details of what we want that element to contain or display, and index [2] is an event handler callback function.

6. Edit the convert function to make sure it will properly convert each sub-array in our vDOM into a DOM element, accounting for the different properties on different element types, as well as setting any event handlers.

7. Declare a function 'updateDOM'. This function will:

   • Update our vDOM variable to be the returned result of invoking createDOM().
   • Use the convert function with our new vDOM details to update jsInput and jsDiv with a new DOM element (linked through a JS object).
   • Replace any children on the body of the DOM with our new elements.
   • Re-set the focus to be on the input if it has been clicked on (feel free to copy this code from the component function where it was given to you).

Now, all of component's functionality should be replaced by the convert and updateDOM functions.

8. Since updateDOM is now the function that updates the DOM with our current data, make sure that it is being called regularly so any data changes will reflect on the DOM.

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Extension Challenge: Adding Nested Elements to the Virtual DOM
What if we wanted more control over placement - what if we had nested elements? Add a few nested elements to the createVDOM function return. How might you adjust your convert function so that it can handle nested elements in the vDOM using semi-visual coding?

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Part 4: Flexible DOM Composition

So far, we have created a "virtual" DOM in JavaScript to be a visual representation of our actual DOM elements, and combined it with one-way data binding. This means our JS data is our "single source of truth" and our vDOM is always consistent with the actual DOM.

Right now, our vDOM is a list of 2 arrays with their element details - but in the "real world" we'll likely have hundreds (or thousands!) of DOM elements to create. We need tools to deal with lists - in this case, we are using an array to create our list.

1. In index.html, edit the script tag to use part4.js as the src file.

In part4.js, we have a vDOM variable (a LIST of subarrays with element details). We also have the same convert function we used before to create a DOM element and corresponding linked JavaScript object with the info in one of our vDOM subarrays. Previously, we manually called convert on vDOM[0] and vDOM[1]. However, if we have hundreds or thousands of DOM elements to create, this approach is not very efficient or flexible.

2. How can we make sure we always run our convert function on each subarray of our vDOM, no matter how many there are or if the number of elements in our vDOM changes? (HINT: since we are working with an array, we can use the map method).

3. Store the resulting list of "converted" JS objects with linked DOM elements in a new variable called 'elems'.

4. Now our new elements exist on the DOM - what do we need to do to render them to the page? How can we make sure each of our elems is appended to the DOM, no matter how many there are?

5. Add 1-2 extra div subarrays to the vDOM and refresh the page - we can now flexibly convert however many elements we have in our vDOM!

With our new "element-flexible" code, we can "compose" our DOM elements.

6. Comment out all of the code at the top of the file, and uncomment all the code below the marked line (this should look familiar - the same createVDOM, updateDOM, and convert functions we used before).

7. Edit the updateDOM function to flexibly handle an unknown amount of elements in the vDOM. Make sure that all of the elems we create are appended to the DOM to replace the previous child elements.

8. Add another element (or a few) to the vDOM in createVDOM - again, no other code should have to change and they should all render properly.

9. For even more flexibility in our code, we can edit our input event handler function to use the event API and target property.

We're getting semi-visual coding - yay!

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Extension Challenges: Additional Functionality

Directives:
Another way that we can give our elements more functionality is by creating functions that take in our element and "decorate it" with added functionality before returning it. In this case, each element on the page has a chance to "do" something in the user's eyes. In reality, that "doing" is happening in JavaScript (e.g. checking a conditional, a loop, etc) and then updating the view (DOM). Try making your elements store some kind of functionality.

Functional Components:
We can also improve our VDOM elements to include additional functionality by creating our elements with a function. This function returns the element out, but before it does, it can run JavaScript code to determine exactly what will be returned. How would you refactor your elements to make them functional in this way? Popular UI frameworks such as React embrace this style of engineering, so if you haven't explored functional components yet, now's your chance!

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Part 5: Hooks & Diffing

Now we've learned to use a "Virtual" DOM for semi-visual coding and we've also created 'element-flexible' code to 'compose' our DOM elements. However, completely rebuilding the entire DOM from scratch every 15 milliseconds makes for terrible performance - as is, our lovely vDOM is quite unusable. We need to add efficiency.

1. In index.html, edit the script tag to use part5.js as the src file.

2. First, let's get rid of the setInterval to avoid repeated function calls to updateDOM.

   • However, we still need to make sure we create our vDOM and populate the DOM with elements on our initial load/render.

We could call updateDOM on every data change (like we did previously inside our event handlers) - but that only works if we actually remember to invoke it everywhere we'd need to, which is unlikely (especially if we have thousands of user actions to handle) - so then our vDOM isn't "real" or guaranteed to accurately reflect our data.

What if, instead of directly updating our data, we run a function to do so?

3. Initialize a function 'updateName' which takes in a value. This function should reassign myName to be the passed-in value.

   • updateName should also make sure the DOM is updated after we update our data.

4. Make sure any part of your code that previously updated myName directly is edited to use updateName instead.

If we only ever update myName with our new updateName function - instead of updating myName directly - we can be sure that our DOM is always updated after a data change.

• We would likely "lock down" myName so that it cannot be accessed directly (or updated from outside of our updater function).

What if we had more data to manage than myName? (Realistically, we would - much more.) Our updateName function is great - but can we refactor it so that it works for any piece of data we might have that our view is dependent on?

5. First, declare a new variable 'data' at the top of our file, and initialize it to an object. Instead of declaring myName separately, nest it inside data as a key-value pair. That way, our data "store" is more flexible and we can add more things to it as needed.

6. Since we want to make the updateName function work with any data, edit it to be called 'updateData' instead. Have it take in a 'label' in addition to the value.

7. updateData should do two things:

   • Update the value on our data object that is stored under the passed in label (or create it if it does not already exist).
   • Update the DOM after our data has been changed.

8. Since we have just implemented changes to the structure of our data, as well as our updateData function, make sure that any place in our code that references our data or updates it is adjusted accordingly.

Now, updateData works to update any piece of data - or state - and so allows us to just "hook" into it. We have a "state hook"!

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Extension Challenge: requestAnimationFrame()
Implement requestAnimationFrame() rather than updateDom directly on data change - so that it never prioritizes over animations (CSS etc).

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With our new updateData hook, we have eliminated unnecessary repeated calls to updateDOM and make sure we are only updating the DOM if our data has changed. However, we are still recreating the entire DOM from scratch every time - even when some of our elements will be rendered exactly the same.

To solve this problem, we can write an 'algorithm' (a series of "smart instructions"), to check what elements actually differ on our updated DOM as compared the the previous DOM - and only change the DOM elements that need to be updated.

So, let's write a diffing algorithm!

9. You're provided with the first half of this function 'findDiff' - go ahead and uncomment it. As you can see, it takes in two parameters, prevVDOM and currentVDOM. Declare 'prevVDOM' globally at the top of the file (but do not initiate it).

   • Inside of findDiff, we're iterating through each element of the currentVDOM array with a for loop.
   • Then, we are using JSON.stringify to check if each element of our currentVDOM matches the corresponding element of prevVDOM or if it has changed. Feel free to read the docs, but don't worry too much about this method right now -- the point of using it here is to compare for equal values, not to check if two objects/arrays are referencing the same place in memory.

10. If a certain element of our currentVDOM has changed (when our data has updated) and does not match what is on our prevVDOM, findDiff should update (or reconcile) the actual DOM element related to that vDOM element.

    • Where have we stored the JavaScript objects we have created with hidden links that allow us to interact with their corresponding nodes on the DOM?
    • How can you make sure you set the correct content on each DOM element based on what's on our new/current vDOM?

Finally, we want to put our new diffing algorithm to good use! In order to implement this, we need to adjust how we update the DOM in order to avoid a full repaint of our view every time.

11. In updateDOM (which will be invoked every time we update our data, via our "state hook"), a couple of things need to happen:
    • If elems (where we store our JS objects that correspond/link to the DOM elements we create) is undefined, we need to create those elements with our vDOM, and then attach them to the DOM.
    • If we already have elems on the DOM, we want to reassign prevVDOM to be a new array, and then spread (hint hint) all of the elements of our existing vDOM into it. Then, we want to update our vDOM with our current data, and use our diffing algorithm to only update the DOM data we need to.

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Congratulations! Starting with breaking down the most granular under-the-hood operations in our UI Hard Parts journey, we have built up to understanding a groundbreaking approach to UI:

• Displayed data/content that our users can interact with (our two goals!).
• Created single source of truth for our data (with one-way data binding).
• Implemented "semi-visual" code with our virtual/visual DOM that allows us to flexibly compose our UI.
• Harnessed techniques for efficiency like state hooks, diffing, and reconciliation.