n/no-sync | Bun Docs

A function such as fs.readFileSync() blocks the thread until it's done. In a server, every other request waits for that one file to be read. The versions that return a promise or take a callback let other work run in the meantime.

The rule only goes by the name. It reports any call to a function or method with a name that ends in Sync. Mentioning the function without calling it isn't reported.

Configure#

Off by default. To turn it on:

bunfig.toml

[lint.rules]
"n/no-sync" = "error" # or "warn"

Every option, set to its default:

bunfig.toml

[lint.rules]
"n/no-sync" = ["error", {
  allowAtRootLevel = false,
  ignores = [], # list of strings
}]

Examples#

Reported:

function handle(request) {
  const page = fs.readFileSync("index.html", "utf8");
  return new Response(page);
}

Not reported:

async function handle(request) {
  const page = await fs.promises.readFile("index.html", "utf8");
  return new Response(page);
}

Options#

Option	Default	What it does
`allowAtRootLevel`	`false`	Allow calls outside of any function, which run once when the file is loaded.
`ignores`	`[]`	Names of functions that are never reported.

const template = fs.readFileSync("index.html", "utf8");

function template() {
  return fs.readFileSync("index.html", "utf8");
}

function hasConfig() {
  return fs.existsSync("config.json");
}

When not to use it#

In scripts, build steps and command line tools, nothing else is waiting for the thread, and the synchronous calls are simpler.