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).
Any reference to an undeclared variable causes a warning, unless the variable is explicitly mentioned in a /*global ...*/
comment, or specified in the globals
key in the configuration file. A common use case for these is if you intentionally use globals that are defined elsewhere (e.g. in a script sourced from HTML).
Examples of incorrect code for this rule:
/*eslint no-undef: "error"*/
var foo = someFunction();
var bar = a + 1;
Examples of correct code for this rule with global
declaration:
/*global someFunction, a*/
/*eslint no-undef: "error"*/
var foo = someFunction();
var bar = a + 1;
Note that this rule does not disallow assignments to read-only global variables. See no-global-assign if you also want to disallow those assignments.
This rule also does not disallow redeclarations of global variables. See no-redeclare if you also want to disallow those redeclarations.
typeof
set to true will warn for variables used inside typeof check (Default false).
Examples of correct code for the default { "typeof": false }
option:
/*eslint no-undef: "error"*/
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.
Examples of incorrect code for the { "typeof": true }
option:
/*eslint no-undef: ["error", { "typeof": true }] */
if(typeof a === "string"){}
Examples of correct code for the { "typeof": true }
option with global
declaration:
/*global a*/
/*eslint no-undef: ["error", { "typeof": true }] */
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.
Examples of correct code for this rule with browser
environment:
/*eslint no-undef: "error"*/
/*eslint-env browser*/
setTimeout(function() {
alert("Hello");
});
Examples of correct code for this rule with node
environment:
/*eslint no-undef: "error"*/
/*eslint-env node*/
var fs = require("fs");
module.exports = function() {
console.log(fs);
};
If explicit declaration of global variables is not to your taste.
This rule provides compatibility with treatment of global variables in JSHint and JSLint.