Disallow Synchronous Methods (no-sync)

In Node.js, most I/O is done through asynchronous methods. However, there are often synchronous versions of the asynchronous methods. For example, fs.exists() and fs.existsSync(). In some contexts, using synchronous operations is okay (if, as with ESLint, you are writing a command line utility). However, in other contexts the use of synchronous operations is considered a bad practice that should be avoided. For example, if you are running a high-travel web server on Node.js, you should consider carefully if you want to allow any synchronous operations that could lock up the server.

Rule Details

This rule is aimed at preventing synchronous methods from being called in Node.js. It looks specifically for the method suffix “Sync” (as is the convention with Node.js operations).

The following patterns are considered warnings:


var contents = fs.readFileSync(somePath).toString();

The following patterns are not considered warnings:


async(function() {
    // ...

When Not To Use It

If you want to allow synchronous operations in your script.


This rule was introduced in ESLint 0.0.9.