Scope

Scope

What it means

Scope determines where a variable is visible and accessible in your code. JavaScript has three levels: global scope (accessible everywhere), function scope (var is confined to the nearest enclosing function), and block scope (let/const are confined to the nearest enclosing {}, including if blocks and loops).

Examples

let globalVar = "I'm global";

function outer() {
  let outerVar = "I'm in outer";

  function inner() {
    let innerVar = "I'm in inner";
    console.log(globalVar);   // accessible -- global scope
    console.log(outerVar);    // accessible -- inner can see outer's scope
    console.log(innerVar);    // accessible -- its own scope
  }

  inner();
  console.log(innerVar);      // ReferenceError -- outer can't see inner's scope
}

if (true) {
  let blockScoped = "only visible in this block";
  var functionScoped = "visible in the whole function/global scope";
}
console.log(functionScoped);   // works -- var ignores block boundaries
console.log(blockScoped);      // ReferenceError -- let respects the block

Common mistake

Assuming var respects block scope like let/const — declaring var inside an if or for block actually attaches it to the surrounding function (or global scope), which can leak variables further than intended.

for (var i = 0; i < 3; i++) {
  // loop body
}
console.log(i);   // 3 -- `i` leaked out of the loop entirely, since var is function-scoped

for (let j = 0; j < 3; j++) {
  // loop body
}
console.log(j);   // ReferenceError -- `j` is properly confined to the loop

Quick practice

1. What’s the key difference between var and let in terms of scope?

   Answer

   var is function-scoped (ignores block boundaries like if and for); let/const are block-scoped, confined to the nearest enclosing {}.

2. Can an inner function access variables declared in its outer (enclosing) function?

   Answer

   Yes — this is lexical scoping. The reverse isn't true: an outer function cannot access variables declared inside an inner function.

3. After a for (var i = ...) loop finishes, is i still accessible outside the loop?

   Answer

   Yes — because var is function-scoped, not block-scoped, i "leaks" into the enclosing function or global scope.