no-undefined

undefined isn't a keyword. It's a global variable, and a function can declare a parameter or a local variable with the same name that holds something else. Code that never writes undefined can't be fooled by that.

The rule reports every place where undefined is read, assigned, or declared.

Configure

Off by default. To turn it on:

[lint.rules]
"no-undefined" = "error" # or "warn"

Examples

Reported:

let selected = undefined;

if (selected === undefined) {
  selected = first();
}

function render(options, undefined) {
  return options.title;
}

Not reported:

let selected;

if (selected == null) {
  selected = first();
}

if (typeof selected === "undefined" || selected === void 0) {
  selected = first();
}

When not to use it

The global undefined is read-only, and no-shadow-restricted-names, which is on by default, reports a local variable with that name. With that rule on, x === undefined is safe, and this rule is a matter of taste.