Disallow Undeclared Variables (no-undef)

Any reference to an undeclared variable causes a warning, unless the variable is explicitly mentioned in a /*global ...*/ comment. This rule provides compatibility with JSHint’s and JSLint’s treatment of global variables.

This rule can help you locate potential ReferenceErrors resulting from misspellings of variable and parameter names, or accidental implicit globals (for example, from forgetting the var keyword in a for loop initializer).

Rule Details


The following code causes 2 warnings, as the globals someFunction and b have not been declared.

/*eslint no-undef: 2*/

var a = someFunction();  /*error 'someFunction' is not defined.*/
b = 10;                  /*error 'b' is not defined.*/

In this code, no warnings are generated, since the global variables have been properly declared in a /*global */ block.

/*global someFunction b:true*/
/*eslint no-undef: 2*/

var a = someFunction();
b = 10;

By default, variables declared in /*global */ are considered read-only. Assignment to a read-only global causes a warning:

/*global b*/
/*eslint no-undef: 2*/

b = 10;                  /*error "b" is read only.*/

Use the variable:true syntax to indicate that a variable can be assigned to.

/*global b:true*/
/*eslint no-undef: 2*/

b = 10;

Explicitly checking an undefined identifier with typeof causes no warning.

/*eslint no-undef: 2*/

if (typeof UndefinedIdentifier === "undefined") {
    // do something ...


You can use this option if you want to prevent typeof check on a variable which has not been declared.

The following patterns are considered problems with option typeof set:

/* eslint no-undef: [2, { typeof: true }] */

if(typeof a === "string"){}      /* error 'a' is not defined. */

The following patterns are not considered problems with option typeof set:

/* eslint no-undef: [2, { typeof: true }] */
/*global a*/

if(typeof a === "string"){}


For convenience, ESLint provides shortcuts that pre-define global variables exposed by popular libraries and runtime environments. This rule supports these environments, as listed in Specifying Environments. A few examples are given below.


Defines common browser globals.

/*eslint-env browser*/

setTimeout(function() {


Defines globals for Node.js development.

/*eslint-env node*/

var fs = require("fs");
module.exports = function() {

When Not To Use It

If explicit declaration of global variables is not to your taste.

Further Reading


This rule was introduced in ESLint 0.0.9.