JavaScript

Variable Declaration
Using Var,let,const
Scope,Hoisting
Re-declaration and Re-assignment

var, let, and const are keywords used for variable declaration in JavaScript. They differ in terms of scope, hoisting, and reassignment capabilities. Here's an explanation of each:

1. var:

   • Variables declared with var are function-scoped or globally scoped, but not block-scoped. This means they are accessible throughout the entire function or globally if declared outside any function.
   • Variables declared with var are hoisted to the top of their scope, which means you can use them before they're declared, but their value will be undefined.
   • var variables can be redeclared and reassigned.

   function example() {
     var x = 10;
     if (true) {
       var x = 20; // This changes the value of the outer x as well
     }
     console.log(x); // 20
   }

2. let:

   • Variables declared with let are block-scoped, which means they're only accessible within the block they're defined in.
   • let variables are not hoisted, so you can't use them before they're declared.
   • let variables can be reassigned but not redeclared within the same scope.

   function example() {
     let y = 30;
     if (true) {
       let y = 40; // This doesn't affect the outer y
       console.log(y); // 40
     }
     console.log(y); // 30
   }

3. const:

   • Variables declared with const are block-scoped, like let.
   • const variables must be assigned a value at the time of declaration and cannot be reassigned.
   • const variables are not hoisted and have the same scoping rules as let.

   function example() {
     const z = 50;
     if (true) {
       const z = 60; // This doesn't affect the outer z
       console.log(z); // 60
     }
     console.log(z); // 50
   }

For modern code, it's generally recommended to use let and const over var. Use let when you need to reassign a variable's value, and use const when the value shouldn't change after assignment. This practice helps improve code readability and maintainability.

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Primitive DataType	->Are Immutable	->Checks Value
Number	String	Boolean
Undefined	Null	Symbol

Primitive data types are the building blocks for more complex data structures and values in JavaScript. Unlike objects, primitive values are immutable, which means they cannot be changed directly; any operation on them creates a new value rather than modifying the existing one. Checks value not reference unlike non-primitive.

In JavaScript, there are six primitive data types:

1. Number: Can be int,float,negative,exponential, "NaN", "Infinity".

   const age = -25;
   const price = -99.99;
   let scientificNum = 1.23e6; // 1.23 * 10^6 (1230000)
   let positiveInfinity = Infinity;
   console.log(positiveInfinity); // Infinity
   console.log(-10 / 0); // -Infinity
   console.log(1e308 * 2); // Infinity
   let result1 = 0 / 0; // NaN(Not a Number)
   let result2 = "abc" * 2; // NaN
   let result3 = Math.sqrt(-1); // NaN
   let nanResult = NaN * 10; // NaN
   let anotherNan = NaN + 5; // NaN

2. String: Represents sequences of characters enclosed in single ('') or double ("") quotes.
   Strings are immutable

   const name = "Alice";
   const message = 'Hello, ' + name;

   templating string:

   console.log(`This template string by ${name}`);
   console.log(`${5+3}`);

3. Boolean: Represents true or false values.

   const isStudent = true;
   const hasPermission = false;

4. Undefined: Represents a variable that has been declared but hasn't been assigned a value.

   let x; // x is undefined

5. Null: Represents the intentional absence of any value or object.

   const noValue = null;

6. Symbol: Represents a unique and immutable value, often used as object property keys.

   const id = Symbol("unique identifier");

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Type Conversions

1. String to Number:

   let str = "42";
   let num = Number(str); // Converts string to number 42

2. Number to String:

   let num = 42;
   let str = String(num); // Converts number to string 42

3. Boolean to String:

   let bool = true;
   let str = String(bool); // Converts boolean to string ("true" or "false")

4. String to Boolean:

   let str = "true";
   let bool = Boolean(str); // Converts string to boolean (true if string is not empty)

5. Number to Boolean:

   let num = 0;
   let bool = Boolean(num); // Converts number to boolean (false if number is 0, true otherwise)

6. Object to String:

   let obj = { key: "value" };
   let str = String(obj); // Converts object to string "[object Object]"

7. String to Object:

   let str = "{ key: 'value' }";
   let obj = JSON.parse(str); // Converts JSON string to object

8. Number to Object:

   let num = 42;
   let obj = new Number(num); // Converts number to Number object

9. Array to String:

   let arr = [1, 2, 3];
   let str = arr.toString(); // Converts array to comma-separated string "1,2,3"

10. String to Array:

    let str = "1,2,3";
    let arr = str.split(","); // Converts comma-separated string to array [1, 2, 3]

parseInt()	Number()

parseInt("123"); // Output: 123
parseInt("010"); // Output: 10 (interpreted as decimal, not octal)
parseInt("0xA"); // Output: 10 (interpreted as decimal)
parseInt("12.34"); // Output: 12 (fractional part is ignored)

Number("123"); // Output: 123
Number("010"); // Output: 10 (interpreted as decimal)
Number("0xA"); // Output: NaN (not a valid decimal number)
Number("12.34"); // Output: 12.34

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