The Power of ‘let’: Understanding Block Scope in JavaScript

The ‘let’ keyword is used for variable declaration in JavaScript. It was introduced in ECMAScript 6 (ES6) as an improvement over the ‘var’ keyword, offering more predictable behavior and improved scoping.

Block Scope with ‘let’:

Variables declared with ‘let’ are block-scoped, meaning they are limited to the block in which they are defined.

A block can be a statement within curly braces ‘{ }’, such as an if statement or a loop. For example

function exampleFunction() {
  if (true) {
    let x = 10; // 'let' variable with block scope
    console.log(x); // Output: 10
  }
  console.log(x); // Throws ReferenceError: x is not defined
}

Block scoping provides better control over variable visibility and reduces the chance of unintentional variable reuse or pollution.

An explanation of why the name let was chosen can be found in the linked StackOverflow answer.

Scoping rules

Variable declared by let have their scope in the block for which they are declared, as well as in any contained sub-blocks. In this way, let works very much like var.

The main difference is that the scope of a ‘var’ variable is the entire enclosing function:

function varTest() {
  var x = 1;
  {
    var x = 2; // same variable!
    console.log(x); // 2
  }
  console.log(x); // 2
}

function letTest() {
  let x = 1;
  {
    let x = 2; // different variable
    console.log(x); // 2
  }
  console.log(x); // 1
}

The Temporal Dead Zone (TDZ):

When using ‘let’ or ‘const’, variables are hoisted to the top of their respective blocks but are not initialized until the actual declaration is encountered. Accessing a variable before its declaration results in a reference error within the Temporal Dead Zone (TDZ). For example:

console.log(a); // Throws ReferenceError: a is not defined
let a = 10;