eslint/no-undefined Restriction

What it does

Disallow the use of undefined as an identifier

Why is this bad?

Using undefined directly can lead to bugs, since it can be shadowed or overwritten in JavaScript. It's safer and more intentional to use null or rely on implicit undefined (e.g., missing return) to avoid accidental issues.

Examples

Examples of incorrect code for this rule:

var foo = undefined;

var undefined = "foo";

if (foo === undefined) {
  // ...
}

function baz(undefined) {
  // ...
}

bar(undefined, "lorem");

Examples of correct code for this rule:

var foo = void 0;

var Undefined = "foo";

if (typeof foo === "undefined") {
  // ...
}

global.undefined = "foo";

bar(void 0, "lorem");

How to use

To enable this rule in the CLI or using the config file, you can use:

CLI

oxlint --deny no-undefined

Config (.oxlintrc.json)

{
  "rules": {
    "no-undefined": "error"
  }
}

References

• Rule Source