fix(curriculum): Update JavaScript Variables and Data Types Review (freeCodeCamp#58679)

Co-authored-by: Jessica Wilkins <67210629+jdwilkin4@users.noreply.github.com>

Author: c0d1ng-ma5ter

curriculum/challenges/english/25-front-end-development/review-javascript-variables-and-data-types/6723be264695fb7e619fe1fa.md

@@ -21,16 +21,16 @@ Data types help the program understand the kind of data it's working with, wheth
 - **Floating point**: A floating point number is a number with a decimal point. Examples include 3.14, 0.5, and 0.0001.
 - **String**: A string is a sequence of characters, or text, enclosed in quotes. `"I like coding"` and `'JavaScript is fun'` are examples of strings.
 - **Boolean**: A boolean represents one of two possible values: `true` or `false`. You can use a boolean to represent a condition, such as `isLoggedin = true`.
-- **Undefined and Null**: An undefined value is a variable that has been declared but not assigned a value. A null value is an empty value, or a variable that has intentionally been assigned a value of `null`.
+- **Undefined and Null**: An `undefined` value is a variable that has been declared but not assigned a value. A `null` value is an empty value, or a variable that has intentionally been assigned a value of `null`.
 - **Object**: An object is a collection of key-value pairs. The key is the property name, and the value is the property value.
 
 Here, the `pet` object has three properties or keys: `name`, `age`, and `type`. The values are `Fluffy`, `3`, and `dog`, respectively.
 
 ```js
 let pet = {
-  name: 'Fluffy',
+  name: "Fluffy",
   age: 3,
-  type: 'dog'
+  type: "dog"
 };
 ```
 
@@ -39,8 +39,8 @@ let pet = {
 In this example below, two symbols are created with the same description, but they are not equal.
 
 ```js
-const crypticKey1= Symbol('saltNpepper');
-const crypticKey2= Symbol('saltNpepper');
+const crypticKey1= Symbol("saltNpepper");
+const crypticKey2= Symbol("saltNpepper");
 console.log(crypticKey1 === crypticKey2); // false
 ```
 
@@ -63,21 +63,21 @@ let cityName;
 - To assign a value to a variable, you can use the assignment operator `=`.
 
 ```js
-cityName = 'New York';
+cityName = "New York";
 ```
 
 - Variables declared using `let` can be reassigned a new value.
 
 ```js
-cityName = 'Los Angeles';
+cityName = "Los Angeles";
 console.log(cityName); // Los Angeles
 ```
 
 - Apart from `let`, you can also use `const` to declare a variable. However, a `const` variable cannot be reassigned a new value.
 
 ```js
-const cityName = 'New York';
-cityName = 'Los Angeles'; // TypeError: Assignment to constant variable.
+const cityName = "New York";
+cityName = "Los Angeles"; // TypeError: Assignment to constant variable.
 ```
 
 - Variables declared using `const` find uses in declaring constants, that are not allowed to change throughout the code, such as `PI` or `MAX_SIZE`.
@@ -103,35 +103,35 @@ let alsoCorrect = "This is also a string";
 - Strings are immutable in JavaScript. This means that once a string is created, you cannot change the characters in the string. However, you can still reassign strings to a new value.
 
 ```js
-let firstName = 'John';
-firstName = 'Jane'; // Reassigning the string to a new value
+let firstName = "John";
+firstName = "Jane"; // Reassigning the string to a new value
 ```
 
 ## String Concatenation in JavaScript
 
 - Concatenation is the process of joining multiple strings or combining strings with variables that hold text. The `+` operator is one of the simplest and most frequently used methods to concatenate strings. 
 
 ```js
-let studentName = 'Asad';
+let studentName = "Asad";
 let studentAge = 25;
-let studentInfo = studentName + ' is ' + studentAge + ' years old.';
+let studentInfo = studentName + " is " + studentAge + " years old.";
 console.log(studentInfo); // Asad is 25 years old.
 ```
 
 - If you need to add or append to an existing string, then you can use the  `+=` operator. This is helpful when you want to build upon a string by adding more text to it over time.
 
 ```js
-let message = 'Welcome to programming, ';
-message += 'Asad!';
+let message = "Welcome to programming, ";
+message += "Asad!";
 console.log(message); // Welcome to programming, Asad!
 ```
 
-- Another way you can concatenate strings is to use the concat method. This method joins two or more strings together.
+- Another way you can concatenate strings is to use the `concat()` method. This method joins two or more strings together.
 
 ```js
-let firstName = 'John';
-let lastName = 'Doe';
-let fullName = firstName.concat(' ', lastName);
+let firstName = "John";
+let lastName = "Doe";
+let fullName = firstName.concat(" ", lastName);
 console.log(fullName); // John Doe
 ```
 
@@ -140,7 +140,7 @@ console.log(fullName); // John Doe
 - The `console.log()` method is used to log messages to the console. It's a helpful tool for debugging and testing your code.
 
 ```js
-console.log('Hello, World!');
+console.log("Hello, World!");
 // Output: Hello, World!
 ```
 
@@ -149,7 +149,7 @@ console.log('Hello, World!');
 - Semicolons are primarily used to mark the end of a statement. This helps the JavaScript engine understand the separation of individual instructions, which is crucial for correct execution.
 
 ```js
-let message = 'Hello, World!'; // first statement ends here
+let message = "Hello, World!"; // first statement ends here
 let number = 42; // second statement starts here
 ```
 
@@ -201,7 +201,7 @@ let isLoggedin = true;
 console.log(typeof isLoggedin); // boolean
 ```
 
-- However, there's a well-known quirk in JavaScript when it comes to null. The `typeof` operator returns `object` for null values.
+- However, there's a well-known quirk in JavaScript when it comes to `null`. The `typeof` operator returns `object` for `null` values.
 
 ```js
 let user = null;