no-sync | Bun Docs

fs.readFileSync() and its relatives block the thread until the work is done. In a server, no other request gets handled in the meantime.

The rule only goes by the name. It reports every object.somethingSync, called or not, no matter what the object is. It doesn't catch a function that's imported by name, such as import { readFileSync } from "node:fs".

ESLint has deprecated this rule. Use n/no-sync instead, which reports those calls too.

Examples#

example.js1 problem

import fs from "node:fs";

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

example.jsNo problems

import fs from "node:fs/promises";

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

This blocks the thread too, but the rule doesn't catch it:

example.jsNo problems

import { readFileSync } from "node:fs";

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

Options#

Option	Default	What it does
`allowAtRootLevel`	`false`	Allow sync methods outside of functions, where they run once as the file loads.

example.jswith { "allowAtRootLevel": true }No problems

import fs from "node:fs";

const config = JSON.parse(fs.readFileSync("config.json", "utf8"));

example.jswith { "allowAtRootLevel": true }1 problem

import fs from "node:fs";

function loadConfig() {
  return JSON.parse(fs.readFileSync("config.json", "utf8"));
}

When not to use it#

In a script or a command-line tool, nothing else is waiting for the thread, and the sync methods are simpler.