block-scoped-var

A var declared inside an if, a loop or a switch is visible in the whole function. Code after the block can still read it, and gets undefined if the block didn't run. This rule treats every var like a let and reports any use outside the block that declares it.

Configure

Off by default. To turn it on:

[lint.rules]
"block-scoped-var" = "error" # or "warn"

Examples

Reported:

function label(user) {
  if (user.admin) {
    var prefix = "Admin: ";
  }
  return prefix + user.name; // "undefinedAda" for everyone else
}

function indexOfFirstEmpty(items) {
  for (var i = 0; i < items.length; i++) {
    if (!items[i]) break;
  }
  return i;
}

Not reported:

function label(user) {
  var prefix = "";
  if (user.admin) {
    prefix = "Admin: ";
  }
  return prefix + user.name;
}

When not to use it

If no-var is on, there's no var left to check, and let and const already behave this way.