JavaScript 4 🧬 How to Variable declaration

4 – How-to Variable declaration

1. How to declare a string variable using single quotes

Steps:

1. Use let or const to declare the variable.
2. Assign a value inside single quotes ('...').
3. The value will be stored as a string type.

let greeting = 'Hello, World!';

---

2. How to declare a string variable using double quotes

Steps:

1. Use let or const to declare the variable.
2. Assign a value inside double quotes ("...").
3. Works similarly to single quotes.

let name = "Alice";

---

3. How to declare a number variable with an integer

Steps:

1. Declare using let, const, or var.
2. Assign an integer value (no decimal point).
3. It will be stored as a number type.

let age = 30;

---

4. How to declare a floating-point number

Steps:

1. Declare using let, const, or var.
2. Assign a value with a decimal point.
3. Stored as a number type.

let height = 5.9;

---

5. How to use underscores in large numbers for readability

Steps:

1. Use underscores (_) between digits.
2. This helps make large numbers more readable.
3. Not used during runtime — just for clarity.

let population = 7_891_024_000;

---

6. How to declare a boolean variable with true

Steps:

1. Declare using let, const, or var.
2. Assign the value true.
3. Boolean values can be either true or false.

let isStudent = true;

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7. How to declare a boolean variable with false

Steps:

1. Declare using let, const, or var.
2. Assign the value false.
3. Used for logical checks in code.

let isLoggedIn = false;

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8. How to declare an undefined variable

Steps:

1. Use let or var.
2. Declare without assigning a value.
3. Its value is automatically set to undefined.

let x;
console.log(x); // undefined

---

9. How to assign null explicitly to a variable

Steps:

1. Use let or const.
2. Assign the keyword null.
3. Represents intentional absence of any object value.

let y = null;

---

10. How to create a symbol using Symbol() constructor

Steps:

1. Use const to declare.
2. Call Symbol() with an optional description.
3. Symbols are unique and immutable.

const id1 = Symbol('id');

---

11. How to declare a BigInt using BigInt() constructor

Steps:

1. Use const.
2. Wrap the number in BigInt(...).
3. For very large integers that exceed normal number limits.

const bigIntExample = BigInt(999_999_999_999_999_999);

---

12. How to declare a BigInt using suffix notation

Steps:

1. Add n at the end of a numeric literal.
2. Ensures it is treated as a BigInt type.

const anotherBigInt = 1234567890123456789012345678901234567890n;

---

13. How to declare an object with properties

Steps:

1. Use let or const.
2. Create an object using curly braces {}.
3. Define keys and values inside.

let person = {
    name: 'Bob',
    age: 25,
};

---

14. How to define a method within an object

Steps:

1. Inside the object, add a key with a function as its value.
2. Use this to refer to the object’s properties.

let person = {
    name: 'Bob',
    greet: function() {
        return "Hello, my name is " + this.name;
    }
};

---

15. How to declare an array using square brackets

Steps:

1. Use let or const.
2. Place elements inside square brackets separated by commas.
3. Arrays can hold different data types.

let fruits = ['apple', 'banana', 'cherry'];

---

16. How to declare a function using function keyword

Steps:

1. Use function followed by the name and parameters.
2. Define what the function does inside curly braces.
3. Return a result if needed.

function multiply(a, b) {
    return a * b;
}

---

17. How to use let for block-scoped variables

Steps:

1. Use let instead of var.
2. The variable is only accessible within the block {}.
3. Introduced in ES6.

if (true) {
    let message = "Hello";
}
// message cannot be accessed outside the if block

---

18. How to declare a constant using const

Steps:

1. Use const to declare a variable.
2. Must initialize with a value at declaration time.
3. Cannot reassign after initial assignment.

const PI = 3.14;
// PI = 3.15; // Error!

---

19. How to use camelCase for naming variables

Steps:

1. Start with lowercase letter.
2. Capitalize the first letter of subsequent words.
3. Avoid spaces or special characters.

let myVariable = "value";

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20. How to avoid starting variable names with digits

Steps:

1. Variables must start with a letter, underscore (_), or dollar sign ($).
2. Never begin with a digit.

let _myVar = 'valid';
let $price = '$5'; // valid too
// let 1variable = "invalid"; // SyntaxError

---

21. How to distinguish between uppercase and lowercase in variable names

Steps:

1. JavaScript is case-sensitive.
2. MyVar, myVar, and MYVAR are all different.

let myVar = 10;
let MyVar = 20;
console.log(myVar); // 10

---

22. How to declare a variable with no initial value

Steps:

1. Declare using let.
2. Leave the assignment part empty.
3. Automatically becomes undefined.

let test;
console.log(test); // undefined

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23. How to combine multiple variables in one declaration

Steps:

1. Use comma-separated assignments with let or const.
2. Useful for grouping related variables.

let a = 1, b = 2, c = 3;

---

24. How to reassign a let variable

Steps:

1. Declare using let.
2. Reassign new values later in the code.
3. Unlike const, this is allowed.

let count = 5;
count = 10; // OK

---

25. How to prevent reassignment with const

Steps:

1. Declare using const.
2. Once assigned, it cannot be changed.
3. Useful for constants like mathematical values.

const MAX_USERS = 100;
// MAX_USERS = 200; // Error!

---

26. How to use var in function scope

Steps:

1. Use var inside functions.
2. Variables are scoped to the function or globally if outside any block.

function example() {
    var localVar = 'Hello';
}
// localVar is not accessible here

---

27. How to use var in global scope

Steps:

1. Declare var outside any function or block.
2. Available throughout the entire script.

var globalVar = 'I am global';

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