Understanding and Fixing the “Uncaught ReferenceError: readLine is not defined” Error in JavaScript

by

When running JavaScript code, encountering an “Uncaught ReferenceError” can be frustrating. This error usually means that you’re trying to use a function or variable that hasn’t been defined. In this post, we’ll address the specific error “Uncaught ReferenceError: readLine is not defined” and provide a solution to fix it.

The Problem

HTML and JavaScript code provided:

<!DOCTYPE html>
<html>
<head>
    <meta charset="ISO-8859-1">
    <title>Quick JavaScript</title>
</head>
<body>
    <h1>Quick Javascript</h1>
    <div id="output">Hello World</div>
    <script>
        var userName = readLine('Who are you?');
        var age = readInt('Please Enter Age');
        var countryOfResidence = readLine('Enter the country where you currently reside');

        var response;

        function checkIfLegalDrinkingAge(age, country) {
            if (country == "USA") {
                response = age > 21 ? true : false;
            } else if (country == "Canada") {
                response = age > 18 ? true : false;
            } else {
                response = 'unknown';
            }
            return response;
        }

        if (response == true) {
            console.log(userName + " resides in " + countryOfResidence + " and is legally allowed to consume alcohol.");
        } else if (response == false) {
            console.log(userName + " resides in " + countryOfResidence + " and is NOT legally allowed to consume alcohol.");
        } else {
            console.log(countryOfResidence + " is unknown. Legal age of consuming alcohol for " + userName + " cannot be determined.");
        }
    </script>
</body>
</html>

Why It’s Not Working

The readLine and readInt functions are not part of standard JavaScript. These functions are likely from a specific environment or library (e.g., a teaching platform like CodeHS) that is not available in a standard browser environment. Hence, when you try to run this code in a typical HTML file, the browser doesn’t know what readLine and readInt are, resulting in the “Uncaught ReferenceError”.

The Solution

To make this code work in a standard browser environment, we need to replace readLine and readInt with standard JavaScript methods to collect user input. We’ll use the prompt function, which displays a dialog box that prompts the user for input.

Here’s the revised code:

<!DOCTYPE html>
<html>
<head>
    <meta charset="ISO-8859-1">
    <title>Quick JavaScript</title>
</head>
<body>
    <h1>Quick Javascript</h1>
    <div id="output">Hello World</div>
    <script>
        var userName = prompt('Who are you?');
        var age = parseInt(prompt('Please Enter Age'), 10);
        var countryOfResidence = prompt('Enter the country where you currently reside');

        var response;

        function checkIfLegalDrinkingAge(age, country) {
            if (country == "USA") {
                response = age > 21 ? true : false;
            } else if (country == "Canada") {
                response = age > 18 ? true : false;
            } else {
                response = 'unknown';
            }
            return response;
        }

        response = checkIfLegalDrinkingAge(age, countryOfResidence);

        if (response === true) {
            console.log(userName + " resides in " + countryOfResidence + " and is legally allowed to consume alcohol.");
        } else if (response === false) {
            console.log(userName + " resides in " + countryOfResidence + " and is NOT legally allowed to consume alcohol.");
        } else {
            console.log(countryOfResidence + " is unknown. Legal age of consuming alcohol for " + userName + " cannot be determined.");
        }
    </script>
</body>
</html>

Explanation:

  • prompt: This function displays a dialog box that prompts the user for input. It returns the text entered by the user.
  • parseInt: This function converts a string to an integer. The second argument, 10, specifies that the number is in base 10.
  • Function checkIfLegalDrinkingAge: This function checks the legal drinking age based on the country.
  • console.log: This outputs messages to the browser’s console.

With these changes, the script should now work in any standard browser environment.

Conclusion

The “Uncaught ReferenceError: readLine is not defined” error occurs because readLine is not a standard JavaScript function. By replacing readLine and readInt with prompt and parseInt, respectively, you can make your script work in a typical HTML page. This solution allows you to collect user input and use it within your JavaScript code effectively.