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variable === undefined vs typeof variable === "undefined"

Written byPhuoc Nguyen
07 Aug, 2020
There are two common ways to check whether a variable is `undefined`. We can use the identical (`===`) or `typeof` operator:
variable === undefined;
typeof variable === 'undefined';


The `typeof` operator works with undeclared variables, while the identical operator will throw a ReferenceError exception.
typeof undeclaredVar === 'undefined'; // true
undeclaredVar === undefined; // throws a ReferenceError exception

Good to know

In the old browsers running ES3 enginee, `undefined` is a global variable name whose primitive value is undefined. The value can be changed:
// ES3
var person = {};
person.name === undefined; // true

// Let's modify the value of undefined
undefined = 'Foo';
person.name === undefined; // false
In order to avoid the issue where `undefined` can be renamed or modified the value, we can wrap the code in an IFFE (immediately invoked function expression) as following:
// It's safe to use undefined here

// Or
In the sample code above, `undefined` is a parameter of function. Since we don't pass any parameter or an undefined variable (`_`) to the function, the parameter will be undefined. This common pattern was used in popular libraries such as jQuery, Backbone, etc.
It's not the case in modern browsers nowadays. From ES5, `undefined` can't be changed because its `Writable` property is set to `false`.

Good practice

Always use `typeof`.

See also

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