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

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:

::: incorrect

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

const foo = someFunction();
const bar = a + 1;

:::

Examples of correct code for this rule with global declaration:

::: correct

/*global someFunction, a*/
/*eslint no-undef: "error"*/

const foo = someFunction();
const 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.

Options

• typeof set to true will warn for variables used inside typeof check (Default false).

typeof

Examples of correct code for the default { "typeof": false } option:

::: correct

/*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:

::: incorrect

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

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

:::

Examples of correct code for the { "typeof": true } option with global declaration:

::: correct

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

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

:::

When Not To Use It

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

Compatibility

This rule provides compatibility with treatment of global variables in JSHint and JSLint.