What are the ways to declare variables in JavaScript?

In JavaScript, variables are containers for storing data values. There are three primary keywords used to declare variables: var, let, and const. Each has its own scope and usage characteristics.

1. `var` Keyword:

- The var keyword was the original way to declare variables in JavaScript. It has function scope, meaning a variable declared with var is accessible throughout the function in which it is declared. If declared outside of any function, it has global scope.

- Variables declared with var can be re-declared and updated within their scope.

- Due to its hoisting behavior and lack of block scope, var can sometimes lead to unexpected behavior and is generally less preferred in modern JavaScript.

- Example:

function exampleVar() {
  var x = 10;
  console.log(x); // Output: 10
  var x = 20; // Re-declaration allowed
  console.log(x); // Output: 20
}
exampleVar();

2. `let` Keyword:

- The let keyword was introduced in ECMAScript 6 (ES6) and provides block scope. This means a variable declared with let is only accessible within the block (e.g., inside an if statement, for loop, or any code block enclosed in curly braces {}) where it is defined.

- Variables declared with let can be updated but not re-declared within their scope.

- let is generally preferred over var for its more predictable scoping behavior.

- Example:

function exampleLet() {
  let y = 30;
  console.log(y); // Output: 30
  y = 40; // Update allowed
  console.log(y); // Output: 40
  // let y = 50; // Error: Identifier 'y' has already been declared
  if (true) {
    let y = 60; // Different scope, no error
    console.log(y); // Output: 60
  }
  console.log(y); // Output: 40
}
exampleLet();

3. `const` Keyword:

- The const keyword, also introduced in ES6, is used to declare constants. Like let, it has block scope.

- Variables declared with const must be initialized with a value at the time of declaration and cannot be re-assigned or re-declared within their scope.

- While the value of a const variable cannot be changed, if the value is an object or array, the properties or elements of that object or array can be modified.

- const is ideal for values that should not change throughout the program.

- Example:

function exampleConst() {
  const z = 70;
  console.log(z); // Output: 70
  // z = 80; // Error: Assignment to constant variable
  // const z = 90; // Error: Identifier 'z' has already been declared
  const obj = { a: 1 };
  obj.a = 2; // Allowed, object properties can be modified
  console.log(obj); // Output: { a: 2 }
}
exampleConst();

In summary, use const by default for values that should not change, let for variables that may need to be updated, and avoid var in modern JavaScript development.

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