Lesson Tuesday

In the last lesson, we focused on the importance of keeping our business logic and UI logic separate. We could've updated our numberOfOccurrencesInText() function to do multiple things but this is a bad practice. A function should just do one thing if possible. We want to have separation of concerns, which means each function is concerned about just one thing and doesn't worry about anything else. That means numberOfOccurrencesInText() just cares about counting the number of occurrences of a substring in a string while boldPassage() should bold matches. Writing a function that did both things wouldn't be good even if it results in fewer lines of code.

In this lesson, we're going to discuss another very important programming concept known as DRY, which means Don't Repeat Yourself. There are a lot of good reasons not to repeat yourself:

  • It results in unnecessary repeated code.
  • It's harder to read and reason about because there's extra repeated code to deal with and read.
  • If the code breaks or it needs to be updated, we have to change it in multiple places, not just one.

If you really practice separating concerns and keeping your code DRY, you are making a huge step towards writing amazing code that clearly communicates your intentions. These are some of the most important techniques you can learn as a coder.

So, let's start by reviewing how DRY our Text Analyzer application is. We'll also discuss tradeoffs and considerations when writing code that's both DRY and separated by concerns. Then, we'll end this lesson by completing our UI logic.

Finding a Balance between DRY Code and Code that Is Separated by Concern


If we review our Text Analyzer scripts, we can find a fair amount of repetition. For example, our two business logic functions and the boldPassage() UI function perform similar tasks:

  • Checking whether the word or text parameters are empty.
  • Splitting the text input into an array.
  • Looping through each element of the text array to perform different actions on it.

As you might guess, sometimes separating our code makes it harder not to repeat ourselves. If we'd just put this all in one function, it would be DRY but have poor separation of logic. With good separation of logic, though, it's not as DRY. Often, the best way to handle this is to extract any repeated code into its own function.

So, let's DRY up our code in a small way: we'll extract the first conditional we use to check whether the word or text parameters are empty into its own function.

First, let's put these functions next to each other. Notice how the conditionals are similar:

// Business Logic

function wordCounter(text) {
  if (text.trim().length === 0) {
    return 0;
  }
  ...
  return wordCount;
}

function numberOfOccurrencesInText(word, text) {
  if (word.trim().length === 0) {
    return 0;
  }
  ...
  return wordCount;
}

function boldPassage(word, text) {
  if ((text.trim().length === 0) || (word.trim().length === 0)) {
    return null;
  }
  ...
  return p;
}

There's two main differences: what each conditional returns and the number of parameters we check. But this won't be a problem — we can easily accommodate these differences.

We'll extract the functionality of the conditional into its own function, and we will put this at the top of our file and call it Utility Logic. You'll see the reason for the name later in this lesson.

scripts.js
// Utility Logic

function isEmpty(testString) {
  return (testString.trim().length === 0);
}

The isEmpty() function returns a boolean. If any inputted string is empty, it will return true. Otherwise, it will return false.

Next, we can plug it into our functions. Here's how we do it:

scripts.js
// Business Logic

function wordCounter(text) {
  if (isEmpty(text)) {
    return 0;
  }
  ...
  return wordCount;
}

function numberOfOccurrencesInText(word, text) {
  if (isEmpty(word)) {
    return 0;
  }
  ...
  return wordCount;
}

function boldPassage(word, text) {
  if (isEmpty(word) || isEmpty(text)) {
    return null;
  }
  ...
  return p;
}

As we can see, instead of checking word.trim().length === 0, our code now checks isEmpty(word).

Hmm... doesn't seem like much of an improvement. Is it really worth it?

Well, imagine if we were using that same code in ten different functions and we realized that we also wanted to account for punctuation. For instance, if someone enters the following: numberOfOccurrencesInText("!", ".");, we want isEmpty() to return false, not true. Would you rather update that code in one place (the isEmpty() function) or in ten different functions? Also, what if in the process of updating the code in ten different places, you missed an eleventh place in the code that needed to be updated as well? These are the sort of scenarios that we need to think about when we're writing JavaScript.

In other situations you'll find that you're able to DRY up a much larger chunk of code. For instance, imagine that we have many different functions that all verify that every input is an actual English word. Imagine that this verification process involves a lot of different steps. Extracting this English-verifying functionality into a separate function that we can then call from any function that needs it makes a lot of sense.

So while refactoring our code to use the isEmpty() function is a very small example of DRYing our code, it illustrates the basic principle of extracting repeated code into its own function.

These kinds of functions are sometimes known as helper or utility functions. You should look for these kinds of opportunities to DRY up your code wherever possible. And again, while the example here is a very small one, it illustrates how we can keep our code DRY with helper functions while still keeping our business and user interface logic separate.

Completing our UI Logic


There's one more thing we need to do to get our application working. We've written our boldPassage() function but we aren't calling it yet. It needs to be called when the form is submitted, so we'll add it to the handleFormSubmission() UI function:

scripts.js
...

function handleFormSubmission() {
  event.preventDefault();
  const passage = document.getElementById("text-passage").value;
  const word = document.getElementById("word").value;
  const wordCount = wordCounter(passage);
  const occurrencesOfWord = numberOfOccurrencesInText(word, passage);
  document.getElementById("total-count").innerText = wordCount;
  document.getElementById("selected-count").innerText = occurrencesOfWord;
  // new lines here!
  let boldedPassage = boldPassage(word, passage);
  if (boldedPassage) {
    document.querySelector("div#bolded-passage").append(boldedPassage);
  } else {
    document.querySelector("div#bolded-passage").innerText = null;
  }
}

window.addEventListener("load", function() {
  document.querySelector("form#word-counter").addEventListener("submit", handleFormSubmission);
});

Look how nice and clean that is! No logic cluttering up this section of the code at all. Instead, it's totally separated out. Even though boldPassage() is a function that deals with UI logic, it doesn't directly alter the DOM. It just returns a paragraph element. That makes it easy to test and easy to separate out. Then we can just call the function when we need it, and then use Element.append() to actually add it to the DOM.

Notice that we only update the DOM if the boldedPassage variable (the result from calling boldPassage()) is truthy. If the boldedPassage variable is null or any other falsey value, the code in the else block will run, setting the contents of the div to null, deleting anything inside of it including any HTML elements.

And with that, we've completely connected all of the logic for our Text Analyzer application. Hooray! Our code is nicely separated and we even added a little utility function to DRY things up a bit.

In a future lesson, we'll create a brand new function for our Text Analyzer application that uses a for loop. We'll also use a for loop when we explore JavaScript's arguments object to expand the capacity of the isEmpty() function.

Visit the cheat sheet to see the completed scripts and HTML for the Text Analyzer application.

Terminology


Don't Repeat Yourself: Known as DRY for short. This is another essential programming concept. We should avoid repeating code where possible. There are a lot of good reasons not to repeat yourself:

  • It results in unnecessary repeated code.
  • It's harder to read and reason about because there's extra repeated code to deal with and read.
  • If the code breaks or it needs to be updated, we have to change it in multiple places, not just one.

Helper/Utility function: A function, often small, with reusable code. We can use helper functions to DRY up our code by removing repeated code, moving it into the helper function, and then calling that helper function wherever that code is needed. Much less repetition!

Code


HTML

<!DOCTYPE html>
<html lang="en-US">
<head>
  <title>Text Analyzer</title>
  <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/[email protected]/dist/css/bootstrap.min.css" integrity="sha384-TX8t27EcRE3e/ihU7zmQxVncDAy5uIKz4rEkgIXeMed4M0jlfIDPvg6uqKI2xXr2" crossorigin="anonymous">
  <link href="css/styles.css" rel="stylesheet" type="text/css">
  <script src="js/scripts.js"></script>
</head>
<body>
  <div class="container">
    <h2>Text Analyzer</h2>
    <form id="word-counter">
      <div class="form-group">
        <p>Input a text passage to get a total word count:</p>
        <textarea id="text-passage" name="text-passage" class="form-control"></textarea>
        <p>Optionally enter a word to count the number of times it occurs in the passage:</p>
        <input type="text" id="word" name="word" class="form-control">
        <br>
        <button type="submit" class="btn btn-success">Submit Survey</button>
      </div>
    </form>
    <p>Total Word Count: <span id="total-count"></span></p>
    <br>
    <p>Selected Word Count: <span id="selected-count"></span></p>
    <div id="bolded-passage">

    </div>
  </div>
</body>
</html>

JS

// Utility Logic

function isEmpty(testString) {
  return (testString.trim().length === 0);
}

// Business Logic

function wordCounter(text) {
  if (isEmpty(text)) {
    return 0;
  }
  let wordCount = 0;
  const textArray = text.split(" ");
  textArray.forEach(function(element) {
    if (!Number(element)) {
      wordCount++;
    }
  });
  return wordCount;
}

function numberOfOccurrencesInText(word, text) {
  if (isEmpty(word)) {
    return 0;
  }
  const textArray = text.split(" ");
  let wordCount = 0;
  textArray.forEach(function(element) {
    if (element.toLowerCase().includes(word.toLowerCase())) {
      wordCount++;
    }
  });
  return wordCount;
}

// UI Logic

function boldPassage(word, text) {
  if (isEmpty(word) || isEmpty(text)) {
    return null;
  }
  const p = document.createElement("p");
  let textArray = text.split(" ");
  textArray.forEach(function(element, index) {
    if (word === element) {
      const bold = document.createElement("strong");
      bold.append(element);
      p.append(bold);
    } else {
      p.append(element);
    }
    if (index !== (textArray.length - 1)) {
      p.append(" ");
    }
  });
  return p;
}

function handleFormSubmission() {
  event.preventDefault();
  const passage = document.getElementById("text-passage").value;
  const word = document.getElementById("word").value;
  const wordCount = wordCounter(passage);
  const occurrencesOfWord = numberOfOccurrencesInText(word, passage);
  document.getElementById("total-count").innerText = wordCount;
  document.getElementById("selected-count").innerText = occurrencesOfWord;
  let boldedPassage = boldPassage(word, passage);
  if (boldedPassage) {
    document.querySelector("div#bolded-passage").append(boldedPassage);
  } else {
    document.querySelector("div#bolded-passage").innerText = null;
  }
}

window.addEventListener("load", function() {
  document.querySelector("form#word-counter").addEventListener("submit", handleFormSubmission);
});

Lesson 31 of 50
Last updated August 10, 2022