Can you explain the scope of variables?

Scope determines where a variable is accessible.

There is, among others:

• global scope,
• function scope,
• block scope.

const globalValue = 1;

function test() {
  const functionValue = 2;

  if (true) {
    const blockValue = 3;
  }
}

globalValue is widely accessible.

functionValue exists only inside the function.

blockValue only inside the {} block.

let and const have block scope.

var has function scope.

Pitfall:

if (true) {
  var x = 10;
}

console.log(x); // 10

but:

if (true) {
  let x = 10;
}

console.log(x); // ReferenceError