no-lone-blocks

Braces that aren't the body of an if, a loop or a function only do something if they contain a let, const or class declaration, or a function declaration in strict mode. Those are scoped to the block. Otherwise the braces change nothing. They're often left over after a condition was deleted, so a reader wonders what's missing.

Configure

Off by default. To turn it on:

[lint.rules]
"no-lone-blocks" = "error" # or "warn"

Examples

Reported:

{
  loadConfig();
}

function start() {
  connect();
  {
    listen();
  }
}

Not reported:

loadConfig();

function start() {
  connect();
  listen();
}

Not reported, because started only exists inside the braces:

{
  const started = Date.now();
  loadConfig();
  console.log(Date.now() - started);
}

Braces around all the statements in a case are fine too:

switch (event.type) {
  case "open": {
    open(event.id);
    break;
  }
}

A block that's the only thing in a function or in another block is reported even if it declares something. The braces around it already limit the scope:

function start() {
  {
    const port = 3000;
    listen(port);
  }
}