This course is an introduction to JavaScript for beginners, it focuses on JavaScript for web development. Learners will explore fundamental concepts using basic JavaScript programs.

JavaScript is a versatile language essential for web development, working alongside HTML and CSS to build both simple and complex web applications. This course offers a comprehensive introduction to JavaScript for beginners, starting with fundamental concepts like problem-solving and simple JavaScript programs. Through interactive exercises, challenges, and quizzes, learners will master control structures, loops, strings, arrays, and functions.
By the end of this course, you'll have solid problem-solving skills, a deep understanding of JavaScript programming, and practical experience in writing clean, efficient code. This knowledge will prepare you for a successful career as a JavaScript developer and significantly enhance your employability in the tech industry.

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# The sentinel value

Sometimes, the loop doesn’t have a fixed number of repetitions. Instead, an indicator value stops the loop. This special value is called the **sentinel value**. For example, we don’t know how much data is in a file without reading it all. However, we know that every file ends with an end-of-file (EOF) mark. So, the EOF mark is the sentinel value in this case.

>**Note:** We should select a sentinel value that’s not expected in the normal input.

# The `while` loop

We use the `while` loop when the termination of the loop depends on the sentinel value instead of a definite number of iterations. 
As a simple example, we want to display the reverse sequence of digits in a positive integer value input by the user.

> **Remember:** For some programs, once you run the program, it will require your input to execute. A pop-up box will appear to type the input in that box first to get the desired results. You must type the inputs one after the other for more than one input.

In the program above:

* In the first line, we take input from the user.

* Followed by the `result` variable to store the subsequent results. 

* The `while` loop statement is followed by a conditional expression and an opening curly bracket (`{`) that indicates the start of the loop.

   > **Note:** There is no semicolon (`;`) at the end of **line 3**.

* The next two statements are in the body of the loop indicated by the indentation.

* The loop body executes if the condition expression `a > 0` is `true`. This statement tests the condition before entering the loop. Therefore, such a loop is called a **pretest loop**.

* The loop terminates when the condition evaluates to `false`.

* In the body of the loop, the first statement stores the value of the unit's position using the modulo operation.

* The second statement strips that units value from `a` using `parseInt()`.

* The closing curly bracket (`}`) indicates the end of the loop, and finally, the last statement logs the result on the console.

Learn and practice loops controlled by a sentinel value.

About the Course

JavaScript Basics

Loops in JavaScript

JavaScript Arrays and Strings

JavaScript Functions

Wrap Up

Sentinel Loops

The sentinel value

The `while` loop

Sentinel Loops

The sentinel value

The while loop

The `while` loop