What is Scope in JavaScript?

Scope in JavaScript refers to the context in which variables are accessible. It determines which part of the code can access and manipulate a particular variable.

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Types of Scope in JavaScript:

1. Global Scope:

• Variables declared outside any function or block.
• Accessible anywhere in the code.

javascriptCopyEditlet globalVar = 'I am global';

function example() {
  console.log(globalVar); // Accessible here
}

console.log(globalVar); // Accessible here too

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2. Local/Function Scope:

• Variables declared inside a function using var, let, or const.
• Accessible only within that function.

javascriptCopyEditfunction example() {
  let localVar = 'I am local';
  console.log(localVar); // Accessible here
}

console.log(localVar); // Error: localVar is not defined

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3. Block Scope (ES6):

• Introduced with let and const.
• Variables declared inside a block {} are only accessible within that block.

javascriptCopyEdit{
  let blockVar = 'I am block-scoped';
  console.log(blockVar); // Accessible here
}

console.log(blockVar); // Error: blockVar is not defined

Note: var does not have block scope. It is function-scoped.

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4. Lexical Scope (Closures):

• A function can access variables from its outer scope where it was defined.
• Inner functions have access to variables of outer functions.

javascriptCopyEditfunction outer() {
  let outerVar = 'I am outer';

  function inner() {
    console.log(outerVar); // Accessible due to lexical scope
  }

  inner();
}

outer();

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Scope Chain:

• When a variable is accessed, JavaScript looks for it in the current scope.
• If not found, it moves up to the outer scope, continuing until it reaches the global scope.
• If still not found, it throws an error.

javascriptCopyEditfunction example() {
  let a = 'local';

  function inner() {
    console.log(a); // Looks for 'a' in inner scope -> not found, moves to outer scope -> found
  }

  inner();
}

example();

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Summary Table:

Scope Type	Declared with	Accessible from
Global Scope	var, let, const	Anywhere in the program
Function Scope	var, let, const	Inside the function
Block Scope	let, const	Inside the block {}
Lexical Scope	(Any)	Inner functions can access outer variables

Understanding scope is crucial to avoid variable conflicts, manage data properly, and write clean, bug-free JavaScript.