Disallow Use of undefined Variable (no-undefined)

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:

Examples

This rule aims to eliminate the use of undefined, and as such, generates a warning whenever it is used.

The following patterns are considered problems:

/*eslint no-undefined: 2*/

var foo = undefined;      /*error Unexpected use of undefined.*/

var undefined = "foo";    /*error Unexpected use of undefined.*/

if (foo === undefined) {  /*error Unexpected use of undefined.*/
    // ...
}

function foo(undefined) { /*error Unexpected use of undefined.*/
    // ...
}

The following patterns are not considered problems:

/*eslint no-undefined: 2*/

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

Version

This rule was introduced in ESLint 0.7.1.

Resources