What is the difference between "let" and "var" keywords in JavaScript?

 In JavaScript, let and var are both used to declare variables, but they have some key differences in terms of their scope and hoisting behavior:

1. Scope: Variables declared with var are function-scoped, while variables declared with let are block-scoped. This means that var variables are accessible throughout the entire function in which they are declared, whereas let variables are only accessible within the block in which they are declared (e.g. a loop, an if statement, or a function).

2. Hoisting: var declarations are hoisted to the top of their containing function or global scope, which means that they are accessible throughout the function or global scope regardless of where they are declared. However, their values are not hoisted, so they will be undefined until they are explicitly assigned a value. let declarations are also hoisted, but unlike var, they are not initialized until they are declared. This means that they cannot be accessed before their declaration statement.

Here's an example to illustrate these differences:

javascript
function myFunction() {
  console.log(x); // Output: undefined
  var x = 'hello';
  console.log(x); // Output: hello

  console.log(y); // Uncaught ReferenceError: y is not defined
  let y = 'world';
  console.log(y); // This line is not executed because of the error above
}

In this example, var x is hoisted to the top of the function, so it can be accessed before it is declared, but its value is undefined until it is assigned 'hello'. In contrast, let y is not hoisted until its declaration statement, so it cannot be accessed before it is declared.

In general, it is recommended to use let and const over var in modern JavaScript code because they provide more predictable scoping behavior and help avoid bugs related to variable hoisting.

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