docs/src/rules/no-undef.md
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:
::: 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.
typeof set to true will warn for variables used inside typeof check (Default false).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"){}
:::
If explicit declaration of global variables is not to your taste.
This rule provides compatibility with treatment of global variables in JSHint and JSLint.