Can you explain the concept of an anonymous block in JavaScript ES6 to me?

Recently, I came across an article on pragmatists which discussed some interesting ES6 features. However, one concept that caught my attention was the use of an anonymous block within a function.

function f() {
  var x = 1
  let y = 2
  const z = 3
  {
    var x = 100
    let y = 200
    const z = 300
    console.log('x in block scope is', x)
    console.log('y in block scope is', y)
    console.log('z in block scope is', z)
  }
  console.log('x outside of block scope is', x)
  console.log('y outside of block scope is', y)
  console.log('z outside of block scope is', z)
}

f()

Answer №1

As stated in the documentation:

The block statement, known as a compound statement in other programming languages, allows multiple statements to be used where only one statement is expected in JavaScript. Grouping statements into blocks is a common practice in JavaScript. Conversely, an empty statement can also be used where no statement is provided even though it's required.

Blocks are frequently paired with if...else and for statements.

Illustrated by an example:

function f() {
  const z = 3
  
    const z = 300
  
  console.log('z outside of block scope is', z)
}

f()

An error occurs without utilizing block scope:

SyntaxError: redeclaration of const z

However, when using block scope:

function f() {
  const z = 3
  {
    const z = 300
    console.log('z in block scope is', z)
  }
  console.log('z outside of block scope is', z)
}

f()

The same code runs smoothly. It's important to note that the block-scoped const z = 300 does not trigger a SyntaxError because it is uniquely declared within the block.

Answer №2

Utilizing anonymous blocks can be advantageous when working with let variables. When using the var keyword, variables are limited to the immediate function body, whereas let variables are restricted to the nearest enclosing block defined by { }.

Answer №3

When you come across a bare block, it's essentially the same as:

for (let i = 0; i < 1; i++) {
  // block contents here
}

or

if (true) {
  // block contents here
}

Although not commonly used, a bare block is nothing extraordinary. The main significance lies in how const and let variables declared within are scoped only to the block itself, rather than the enclosing function.

The behavior mirrors other non-function blocks in Javascript - similar to an if block, while block, or for block. However, unlike these, a bare block doesn't require a condition to enter and executes just once.

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