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

Understanding the Differences between "let" and "var" in JavaScript

The introduction of the "let" keyword in ECMAScript 6 has sparked confusion regarding its relationship with the traditional "var" keyword. To clarify, "let" introduces local variables with block scoping, while "var" variables are function scoped.

Block Scoping vs. Function Scoping

The primary distinction between "let" and "var" lies in their scoping mechanism. "Var" variables are declared within the function body, regardless of curly braces delimiting blocks. This means they can be accessed from anywhere within the function. In contrast, "let" variables are confined to the block in which they are declared.

Code Example

Consider the following code:

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 "run" function, even within the inner block. On the other hand, "bar" (declared with "let") is only available within the block where it is declared. This distinction highlights the block scoping of "let" variables.

When to Use "let" vs. "var"

Given the different scoping mechanisms, it is prudent to use "let" in situations where it is necessary to restrict variable visibility to specific blocks. This is particularly useful for reducing variable pollution and preventing accidental overwriting.

Conversely, "var" remains appropriate in scenarios where function-wide accessibility is desired, such as declaring global variables or when using older codebases that rely on "var" for variable declaration.

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