Disallow Use of undefined
Variable (no-undefined)
The undefined
variable in JavaScript is actually a property of the global object. As such, in ECMAScript 3 it was possible to overwrite the value of undefined
. While ECMAScript 5 disallows overwriting undefined
, it's still possible to shadow undefined
, such as:
function doSomething(data) {
var undefined = "hi";
// doesn't do what you think it does
if (data === undefined) {
// ...
}
}
Because undefined
can be overwritten or shadowed, reading undefined
can give an unexpected value. (This is not the case for null
, which is a keyword that always produces the same value.) To guard against this, you can avoid all uses of undefined
, which is what some style guides recommend and what this rule enforces. Those style guides then also recommend:
- Variables that should be
undefined
are simply left uninitialized. (All uninitialized variables automatically get the value ofundefined
in JavaScript.) - Checking if a value is
undefined
should be done withtypeof
. - Using the
void
operator to generate the value ofundefined
if necessary.
As an alternative, you can use the no-global-assign and no-shadow-restricted-names rules to prevent undefined
from being shadowed or assigned a different value. This ensures that undefined
will always hold its original, expected value.
Rule Details
This rule aims to eliminate the use of undefined
, and as such, generates a warning whenever it is used.
Examples of incorrect code for this rule:
/*eslint no-undefined: "error"*/
var foo = undefined;
var undefined = "foo";
if (foo === undefined) {
// ...
}
function foo(undefined) {
// ...
}
Examples of correct code for this rule:
/*eslint no-undefined: "error"*/
var foo = void 0;
var Undefined = "foo";
if (typeof foo === "undefined") {
// ...
}
global.undefined = "foo";
When Not To Use It
If you want to allow the use of undefined
in your code, then you can safely turn this rule off.
Further Reading
- undefined - JavaScript | MDN
- Understanding JavaScript’s ‘undefined’ | JavaScript, JavaScript...
- ECMA262 edition 5.1 §15.1.1.3: undefined
Related Rules
Version
This rule was introduced in ESLint 0.7.1.