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 of undefined in JavaScript.)
• Checking if a value is undefined should be done with typeof.
• Using the void operator to generate the value of undefined 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

• no-undef-init
• no-void
• no-shadow-restricted-names
• no-global-assign

Version

This rule was introduced in ESLint 0.7.1.

Resources

• Rule source
• Documentation source