variable === undefined vs typeof variable === "undefined"
Written byPhuoc Nguyen
Created
07 Aug, 2020
Category
JavaScript
There are two common ways to check whether a variable is
`undefined`. We can use the identical (`===`) or `typeof` operator:js
variable === undefined;
typeof variable === 'undefined';#Difference
The
`typeof` operator works with undeclared variables, while the identical operator will throw a ReferenceError exception.js
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:js
// ES3
var person = {};
person.name === undefined; // true
// Let's modify the value of undefined
undefined = 'Foo';
person.name === undefined; // falseIn 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:js
(function(undefined){
// It's safe to use undefined here
})();
// Or
(function(undefined){
...
})(_);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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Phước Nguyễn