Let vs. Var in JavaScript: Demystifying Scope and Temporal Dead Zones

Introduced in ECMAScript 6, the let statement has sparked confusion among developers, particularly how it differs from the established var keyword. This article delves into the nuances between these two variable declarations, highlighting their scoping rules and best use cases.

Scope

The fundamental distinction lies in their scoping behavior. Variables declared with var are restricted to the function they're defined in (function scope), while let variables are confined to the block they're enclosed within (block scope). This means let variables have a much narrower scope, preventing them from interacting with variables defined outside their immediate block.

Example:

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 the example above, the var variable moo remains accessible even outside its block, while the let variable baz throws a ReferenceError when accessed outside its block.

Temporal Dead Zones

Another difference is temporal dead zones (TDZ). For let variables, there exists a TDZ from the point of declaration until the end of the block they're defined in. During this period, accessing a let variable without initialization results in a ReferenceError.

Example:

function test() {
  console.log(foo); // ReferenceError
  let foo = "Bar";
}

test();

In this code, accessing the let variable foo before it's initialized results in a ReferenceError because of the TDZ.

When to Use Let vs. Var

In general, prefer using let for variables within blocks to avoid polluting the global scope and prevent variable name collisions. Var is still useful for variables that need to be accessible across multiple blocks or functions.