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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;

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;

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";

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

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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