JavaScript Basics

console.log() is a function in JavaScript that outputs a message to the web console. It's one of the most basic and commonly used debugging tools.

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

Hello, World!

This is the simplest use of console.log(). It's often used as a first step in learning a new programming language. In JavaScript, you can use it to quickly check if your code is running or to see the value of a variable at a specific point in your program.

console.log() can take multiple arguments, which will be printed out separated by spaces.

console.log('Name:', 'Alice', 'Age:', 30);

Name: Alice Age: 30

When you pass multiple arguments to console.log(), it will print them all out in order, separated by spaces. This is useful for logging multiple values at once or for adding labels to your log outputs to make them more readable.

One of the most common uses of console.log() is to output the value of variables for debugging purposes.

let x = 5;
console.log('The value of x is:', x);

The value of x is: 5

Here, we're using console.log() to print out both a string and the value of a variable. This is extremely useful when debugging, as it allows you to see the current state of your variables at different points in your code execution.

To create a variable using 'var' in JavaScript, you declare it with the 'var' keyword. 'var' creates variables that are either function-scoped or globally-scoped.

Steps

• Use the 'var' keyword
• Choose a descriptive variable name
• Optionally, initialize the variable with a value
• Be aware of function or global scope

Good Example

// Using var (function-scoped or globally-scoped)
var x = 10;
if (true) {
  var x = 20; // This will overwrite the previous x
}
console.log(x); // Outputs 20

// Caution: var can lead to unexpected behavior due to hoisting
console.log(y); // Outputs undefined, not an error
var y = 5;

Bad Example

// Incorrect usage of var
var 123abc = "invalid name"; // Invalid: starts with a number
var function = "reserved word"; // Invalid: uses a reserved keyword
var globalVar = "avoid globals"; // Caution: creates a global variable

Tips

• Avoid using 'var' in modern JavaScript; prefer 'let' or 'const'
• Be cautious of function scope and hoisting behavior
• Don't rely on global variables created with 'var'
• Use meaningful variable names

To create a variable using 'let' in JavaScript, you declare it with the 'let' keyword. 'let' creates block-scoped variables.

Steps

• Use the 'let' keyword
• Choose a descriptive variable name
• Optionally, initialize the variable with a value
• Remember that 'let' is block-scoped
• Use 'let' for variables that will be reassigned

Good Example

// Using let (block-scoped)
let count = 0;
if (true) {
  let count = 1; // This creates a new variable, doesn't affect outer count
  console.log(count); // Outputs 1
}
console.log(count); // Outputs 0

// Proper use: let for variables that will be reassigned
let score = 75;
score = score + 25; // Now score is 100
console.log(score); // Outputs 100

Bad Example

// Incorrect usage of let
let x = 5;
let x = 10; // Error: can't redeclare block-scoped variable
{
  console.log(y); // Error: can't use before declaration
  let y = 20;
}
console.log(y); // Error: y is not defined in this scope

Tips

• Use 'let' for variables that will be reassigned
• Remember that 'let' is block-scoped
• Don't redeclare variables with 'let' in the same scope
• Initialize 'let' variables before using them