`let` vs. `var` in JavaScript: What are the Key Differences in Variable Scope?

"let" vs. "var" in JavaScript: What's the Difference?

JavaScript introduced the let keyword in ECMAScript 6, offering a more granular scope for variables compared to the traditional var.

Differences in Scope

The primary difference lies in variable scoping. Variables declared with var have function-level scope, meaning they can be accessed anywhere within the enclosing function. On the other hand, variables declared with let have block-level scope, meaning they can only be accessed within the immediate block in which they are defined.

When to Use "let" Instead of "var"

Block Scoping:

• When you need to create a variable that is only accessible within a specific part of your code.
• To avoid variable redeclaration or Shadowing within the same scope.

Temporal Dead Zone:

• Variables declared with let exist in a "temporal dead zone" before their declaration, causing any attempts to access them before definition to result in a ReferenceError. This helps prevent accidental variable usage.

Example Usage

Consider the following code snippet:

function run() {
  var foo = "Foo";
  let bar = "Bar";

  console.log(foo, bar); // Foo Bar

  {
    var moo = "Mooo"
    let baz = "Bazz";
    console.log(moo, baz); // Mooo Bazz
  }

  console.log(moo); // Mooo
  console.log(baz); // ReferenceError
}

run();

In this example, foo (declared with var) is accessible throughout the function, while bar (declared with let) is only accessible within the nested block. Attempting to access baz outside the block results in a ReferenceError due to its block-level scope.

Note: let is preferred over var in most modern JavaScript development due to its stricter scoping and enhanced code readability.

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