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).

Options

This rule has an optional object option { allowAtRootLevel: <boolean> }, which determines whether synchronous methods should be allowed at the top level of a file, outside of any functions. This option defaults to false.

Examples of incorrect code for this rule with the default { allowAtRootLevel: false } option:

/*eslint no-sync: "error"*/

fs.existsSync(somePath);

function foo() {
  var contents = fs.readFileSync(somePath).toString();
}

Examples of correct code for this rule with the default { allowAtRootLevel: false } option:

/*eslint no-sync: "error"*/

obj.sync();

async(function() {
    // ...
});

Examples of incorrect code for this rule with the { allowAtRootLevel: true } option

/*eslint no-sync: ["error", { allowAtRootLevel: true }]*/

function foo() {
  var contents = fs.readFileSync(somePath).toString();
}

var bar = baz => fs.readFileSync(qux);

Examples of correct code for this rule with the { allowAtRootLevel: true } option

/*eslint no-sync: ["error", { allowAtRootLevel: true }]*/

fs.readFileSync(somePath).toString();

When Not To Use It

If you want to allow synchronous operations in your script, do not enable this rule.

Version

This rule was introduced in ESLint 0.0.9.

Resources

• Rule source
• Documentation source