The undefined
variable is unique in JavaScript because it 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) {
// ...
}
}
This represents a problem for undefined
that doesn't exist for null
, which is a keyword and primitive value that can neither be overwritten nor shadowed.
All uninitialized variables automatically get the value of undefined
:
var foo;
console.log(foo === undefined); // true (assuming no shadowing)
For this reason, it's not necessary to explicitly initialize a variable to undefined
.
Taking all of this into account, some style guides forbid the use of undefined
, recommending instead:
- Variables that should be
undefined
are simply left uninitialized. - Checking if a value is
undefined
should be done withtypeof
. - Using the
void
operator to generate the value ofundefined
if necessary.
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";
If you want to allow the use of undefined
in your code, then you can safely turn this rule off.