Writing first JavaScript program

Introduction

Writing your first JavaScript program helps you learn how to tell the computer what to do using code. It is the first step to creating interactive web pages.

You want to show a message on a web page.

You want to test if your computer can run JavaScript code.

You want to learn how to write simple instructions for the browser.

You want to start making websites that respond to user actions.

Syntax

Javascript

console.log('Hello, world!');

console.log() is used to show messages in the browser's console.

The text inside quotes will be shown exactly as written.

Examples

This prints the message 'Hello, world!' in the console.

Javascript

console.log('Hello, world!');

You can change the message to anything you want.

Javascript

console.log('Welcome to JavaScript!');

You can also print numbers, not just text.

Javascript

console.log(123);

Sample Program

This program prints a simple message to the console to show that JavaScript is working.

Javascript

console.log('My first JavaScript program');

Important Notes

To see the output, open your browser's developer tools and look at the console tab.

Every JavaScript statement ends with a semicolon ;, but it is optional in many cases.

Summary

JavaScript programs tell the browser what to do.

console.log() shows messages in the console.

Your first program is a simple message to check everything works.

Practice

1. What does the console.log() function do in JavaScript?

easy

A. It changes the background color of the page.

B. It creates a pop-up alert on the screen.

C. It stops the program from running.

D. It shows messages in the browser console.

Solution

1. Step 1: Understand the purpose of console.log()

   This function is used to display messages or values in the browser's console, which helps developers see output or debug.

2. Step 2: Compare with other options

   Options A, B, and D describe different actions that console.log() does not perform.

3. Final Answer:

   It shows messages in the browser console. -> Option D

4. Quick Check:

   console.log() shows messages [OK]

Hint: Remember: console.log prints messages to the console [OK]

Common Mistakes:

• Thinking console.log creates alerts
• Confusing console.log with styling commands
• Assuming console.log stops the program

2. Which of the following is the correct syntax to print "Hello, World!" in JavaScript?

easy

A. print("Hello, World!");

B. console.log("Hello, World!");

C. echo "Hello, World!";

D. System.out.println("Hello, World!");

Solution

1. Step 1: Identify JavaScript syntax for output

   JavaScript uses console.log() to print messages to the console.

2. Step 2: Check other options for language mismatch

   A is Python style, B is Java style, C is PHP style, so they are incorrect for JavaScript.

3. Final Answer:

   console.log("Hello, World!"); -> Option B

4. Quick Check:

   JavaScript output uses console.log() [OK]

Hint: Use console.log() to print messages in JavaScript [OK]

Common Mistakes:

• Using print() instead of console.log()
• Confusing JavaScript with other languages
• Missing parentheses or semicolon

3. What will be the output of the following code?

console.log('Hi');
console.log('There');

medium

A. Hi
There

B. Hi There

C. Hi\nThere

D. HiThere

Solution

1. Step 1: Understand console.log behavior

   Each console.log() prints its message on a new line in the console.

2. Step 2: Analyze the output lines

   The first prints "Hi", then the second prints "There" on the next line, so output is two lines: "Hi" then "There".

3. Final Answer:

   Hi There -> Option A

4. Quick Check:

   console.log prints each message on a new line [OK]

Hint: Each console.log prints on a new line automatically [OK]

Common Mistakes:

• Thinking console.log joins messages on one line
• Confusing \n as literal text instead of newline
• Ignoring line breaks between calls

4. Find the error in this JavaScript code:

console.log('Hello World')

medium

A. console.log is misspelled

B. Incorrect quotes used

C. No error, code is correct

D. Missing semicolon at the end

Solution

1. Step 1: Check syntax correctness

   The code uses correct function name, quotes, and parentheses.

2. Step 2: Consider JavaScript semicolon rules

   Semicolons are optional at the end of statements in JavaScript, so missing semicolon is not an error.

3. Final Answer:

   No error, code is correct -> Option C

4. Quick Check:

   JavaScript allows optional semicolons [OK]

Hint: Semicolons are optional but recommended; no error here [OK]

Common Mistakes:

• Thinking missing semicolon causes error
• Confusing quote types as error
• Assuming console.log is misspelled

5. You want to write a JavaScript program that prints your name and age on separate lines. Which code will do this correctly?

hard

A. console.log('Name: John'); console.log('Age: 25');

B. console.log('Name: John\nAge: 25');

C. console.log('Name: John' + 'Age: 25');

D. console.log('Name: John Age: 25');

Solution

1. Step 1: Understand printing on separate lines

   Using two console.log() calls prints each message on its own line.

2. Step 2: Check other options for line breaks

   console.log('Name: John\nAge: 25'); uses \n which creates a newline in the output; console.log('Name: John' + 'Age: 25'); joins to 'Name: JohnAge: 25' on one line; console.log('Name: John Age: 25'); prints all on one line.

3. Final Answer:

   console.log('Name: John'); console.log('Age: 25'); -> Option A

4. Quick Check:

   Separate console.log calls print on separate lines [OK]

Hint: Use separate console.log() for each line [OK]

Common Mistakes:

• Using + without spaces causing joined text
• Thinking \n creates a newline (prints literal \n)
• Printing all info in one console.log without breaks