![Avatar image for developer.mozilla.org](https://developer.mozilla.org/favicon-48x48.cbbd161b.png)
no-undefined
Disallow the use of undefined
as an identifier
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 = ;
var = "foo";
if (foo === ) {
// ...
}
function baz() {
// ...
}
bar(, "lorem");
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";
bar(void 0, "lorem");
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.
Related Rules
Version
This rule was introduced in ESLint v0.7.1.
Further Reading
![Avatar image for developer.mozilla.org](https://developer.mozilla.org/favicon-48x48.cbbd161b.png)