What are the Key Differences Between JavaScript's `let` and `var` Keywords?

Deciphering the Differences Between "let" and "var" in JavaScript

ES6 introduced the "let" keyword, often referred to as a local variable. Its behavior differs significantly from the traditional "var" keyword, and understanding these differences is crucial for effective JavaScript coding.

Scope and Block Boundaries

The primary distinction lies in scoping rules. "var" variables are associated with the enclosing function scope, while "let" variables are bound to the nearest enclosing block (denoted by {})—also known as the block scope.

Example Code

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();

1. Function Scope (var): "foo" declared with "var" is accessible throughout the "run()" function.
2. Block Scope (let): "bar" declared with "let" is only available within the block it is defined.
3. Redeclaration (var): "moo" declared with "var" can be redeclared within the block, leading to shadowing.
4. Temporal Dead Zone (let): "baz" declared with "let" becomes accessible only after the block is executed, creating a temporal dead zone.
5. Hoisting (var): "moo" is hoisted and can be accessed outside the block it is declared in, but "baz" is not hoisted.

When to Use "let" Instead of "var"

"let" should be used when:

• You need variables scoped to a specific block.
• You want to avoid name conflicts and shadowing.
• You want to implement block-level iteration (e.g., "for let" loops).

"var" can still be used when:

• You need variables accessible throughout a function.
• You need to redeclare variables within nested blocks.
• You are working with legacy code that heavily relies on "var".

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