#no-sync

#Configuration

import { defineConfig } from '@rslint/core';

export default defineConfig([
  {
    plugins: ['node'],
    rules: {
      'node/no-sync': 'error',
    },
  },
]);

Disallow synchronous calls identified by names ending in Sync.

Synchronous I/O can block other work in a Node.js application. Prefer an asynchronous API where the surrounding code can wait for its result.

#Examples

// Incorrect
fs.readFileSync(path);
readFileSync(path);
readFileSync.call(null, path);

// Correct
await fs.promises.readFile(path);

The rule checks names without requiring a Node.js import. It also checks direct identifier arguments, such as consume(readFileSync), and identifiers inside a member-expression callee. String keys such as fs['readFileSync']() are allowed. The rule provides no automatic fixes or suggestions.

#Options

#allowAtRootLevel

Set allowAtRootLevel to true to allow synchronous calls outside functions. The default is false. Top-level blocks and class field initializers are also outside functions; method bodies and function parameter defaults are inside.

// With { allowAtRootLevel: true }
const config = fs.readFileSync(path); // Allowed

function reload() {
  return fs.readFileSync(path); // Reported
}

#ignores

The default is []. Strings ignore exact identifier names:

export default [{
  plugins: ['node'],
  rules: {
    'node/no-sync': ['error', { ignores: ['readFileSync'] }],
  },
}];

Object entries use TypeScript declaration information:

Use / separators in file patterns on every platform. Backslashes escape glob characters. Package patterns are JavaScript regular expressions in Unicode mode; invalid patterns, such as (, are configuration errors.

Each object can include a name array. These names refer to the declared type, including its containing class or interface, such as { from: 'lib', name: ['CSSStyleSheet.replaceSync'] }. Imported aliases use the original declaration name. Omit name to ignore all matching declarations.

#Differences from upstream

• When type information is unavailable, object entries in ignores suppress nothing; string entries still apply. Upstream stops with an error in this case.
• An empty file pattern ignores no declarations. Upstream stops with an error.
• File patterns such as ./helpers.ts also work on Windows and in working directories containing uppercase letters on case-insensitive file systems. Upstream can fail to ignore matching declarations in these cases.
• Directory checks distinguish siblings with a shared prefix. For example, with typeRoots: ['./types'], { from: 'file' } can ignore declarations in types-extra/helper.ts; upstream incorrectly excludes them. A file glob cannot match declarations in a sibling project-extra directory when the working directory is project, even though upstream may accept them.
• Package patterns are validated before linting. Upstream may only report an invalid pattern when it encounters a matching call. Long Unicode property names such as \p{Letter} are unsupported; use \p{L} instead.

Some file globs match differently. The table shows whether each pattern ignores an example declaration in a case-sensitive project directory. Prefer explicit paths, **/foo.ts, or alternatives such as **/*.{ts,tsx} when sharing a configuration with upstream.

#Original documentation

• eslint-plugin-n: no-sync
• Source code