PRDV401: Introduction to JavaScript I

Block scoping rules with var or function declaration in non-strict mode

Variables declared with var or created by function declarations in non-strict mode do not have block scope. Variables introduced within a block are scoped to the containing function or script, and the effects of setting them persist beyond the block itself. In other words, block statements do not introduce a scope. For example:

var x = 1;
{
  var x = 2;
}
console.log(x); // logs 2

This logs 2 because the var x statement within the block is in the same scope as the var x statement before the block.

In non-strict code, function declarations inside blocks behave strangely. Do not use them.

Block scoping rules with let, const or function declaration in strict mode

By contrast, identifiers declared with let and const do have block scope:

let x = 1;
{
  let x = 2;
}
console.log(x); // logs 1

The x = 2 is limited in scope to the block in which it was defined.

The same is true of const:

const c = 1;
{
  const c = 2;
}
console.log(c); // logs 1 and does not throw SyntaxError...

Note that the block-scoped const c = 2 does not throw a SyntaxError: Identifier 'c' has already been declared because it can be declared uniquely within the block.

In strict mode, starting with ES2015, functions inside blocks are scoped to that block. Prior to ES2015, block-level functions were forbidden in strict mode.