JavaScript undefined

In JavaScript, a variable without a value has the value undefined.

The type is also undefined.

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Undefined Variables

A variable declared without a value will automatically have the value undefined.

Example

let car;

document.getElementById("demo").innerHTML = car;

The Type of undefined

You can use typeof to check if a variable is undefined.

Example

let car;

document.getElementById("demo").innerHTML = typeof car;

Empty Values

An empty string has a value and a type.

It is not the same as undefined.

Example

let text = "";

document.getElementById("demo").innerHTML =

text + " " + typeof text;

Objects and undefined

Accessing a non-existing object property returns undefined.

Example

const person = {firstName:"John", lastName:"Doe"};

document.getElementById("demo").innerHTML = person.age;

Functions and undefined

A function without a return value returns undefined.

Example

function myFunction() {

  let x = 5;

}

document.getElementById("demo").innerHTML = myFunction();

You Can Empty an Object

Objects can be emptied by setting the value to undefined.

Example

const person = {firstName:"John", lastName:"Doe"};

person = undefined;

Objects can also be emptied by setting the value to null.

Note

undefined is a JavaScript value.

It means that a variable has been declared, but no value has been assigned.