Unexpected sync method: '{a}'

When do I get this error?

The "Unexpected sync method: '{a}'" error is thrown when JSLint (versions from March 2012 onwards) encounters an attempt to access a property whose identifier ends with the character sequence Sync. In the following Node.js example we attempt to get an array containing the names of files within a directory:

/*jslint node: true */
var fs = require("fs"),
    files = fs.readdirSync("myDirectory");

Note that although the error messages states "method", JSLint will actually raise this error for an attempt to access any property that fits the criteria, whether it's a method or not (it doesn't check to see if a pair of invoking parentheses follow the identifier).

Why do I get this error?

This error is raised to highlight a lack of convention and possible bad practice. Your code should run without problems if you don't change it, but it's likely there are better ways to acheive the same result.

In Node.js in particular, there are many asynchronous methods that provide synchronous equivalents. For example, there is a readdir method that is the asynchronous version of the [readdirSync][readdirsync] method in our example above.

In almost all situations it's preferrable to use the asynchronous method over the synchronous one. The reason is that synchronous methods will block execution until they have finished doing whatever they need to do. In the browser this can result in an apparently "frozen" page. To fix this issue, simply rework your code to use the asynchronous version of the method:

/*jslint node: true */
var fs = require("fs");

fs.readdir("myDirectory", function (err, files) {
    "use strict";
    if (!err) {

Alternatively, if you have a real need to use synchronous methods, you can surpress this error by setting the stupid option to true. You can tell by the name of the option how Douglas Crockford, author of JSLint, feels about this:

/*jslint node: true, stupid: true */
var fs = require("fs"),
    files = fs.readdirSync("myDirectory");

About the author

James Allardice

This article was written by James Allardice, Software engineer at Tesco and orangejellyfish in London. Passionate about React, Node and writing clean and maintainable JavaScript. Uses linters (currently ESLint) every day to help achieve this.