JavaScript typeof

The typeof operator in JavaScript is used to ascertain the data type of a value or expression. The type of the operand is represented by a string that the typeof operator returns.

There are 5 distinct data types in JavaScript that may hold values:

string
number
boolean
object
function

There are 6 different kinds of objects:

Object
Date
Array
String
Number
Boolean

There are only two data types that are valueless.

null
undefined

The typeof Operator

The typeof operator can be used to determine a JavaScript variable's data type.

Example

<!DOCTYPE html>
<html>
<body>

<h1>The typeof Operator</h1>
<script>
    document.write(
        "'Peter' is " + typeof "Peter" + "<br>" +
        "4.15 is " + typeof 4.15 + "<br>" +
        "NaN is " + typeof NaN + "<br>" +
        "false is " + typeof false + "<br>" +
        "[0, 1, 2, 3] is " + typeof [0, 1, 2, 3] + "<br>" +
        "{name:'Peter', age:26} is " + typeof {name:'Peter', age:26} + "<br>" +
        "new Date() is " + typeof new Date() + "<br>" +
        "function () {} is " + typeof function () {} + "<br>" +
        "myBike is " + typeof myBike + "<br>" +
        "null is " + typeof null
    );
</script>

</body>
</html>

Preview

Primitive Data

A single basic data item without any extra characteristics or methods is referred to as a primitive data value.

One of these basic kinds can be obtained by using the typeof operator:

string
number
boolean
undefined

Example

<!DOCTYPE html>
<html>
<body>

<h1>JavaScript Primitive Data</h1>
<script>
    document.write(
        typeof "Peter" + "<br>" +
        typeof 4.15 + "<br>" +
        typeof true + "<br>" +
        typeof false + "<br>" +
        typeof z
    );
</script>

</body>
</html>

Preview

Complex Data

One of two complex types can be returned by the typeof operator:

function
object

In the case of objects, arrays, and null, the typeof operation returns "object". For functions, the typeof operator does not return "object".

Example

<!DOCTYPE html>
<html>
<body>

<h1>JavaScript Complex Data</h1>
<script>
    document.write(
        typeof {name:'Peter', age:26} + "<br>" +
        typeof [0,1,2,3,4] + "<br>" +
        typeof null + "<br>" +
        typeof function myFunction(){}
    );
</script>

</body>
</html>

Preview

The Data Type of typeof

An operator's type is not a variable. An operator, that is. There is no data type for the operators (+ - * /). The typeof operation, however, always yields a string (containing the type of the operand).

The constructor Property

All JavaScript variables' constructor functions are returned through the constructor property. The constructor attribute is used to determine if an object is a Date or an Array (contains the word "Array") (contains the word "Date").

Example

<!DOCTYPE html>
<html>
<body>

<h1>JavaScript constructor Property</h1>
<script>
    document.write(
        "Peter".constructor + "<br>" +
        (4.15).constructor + "<br>" +
        false.constructor + "<br>" +
        [0,1,2,3,4].constructor + "<br>" +
        {name:'Peter', age:26}.constructor + "<br>" +
        new Date().constructor + "<br>" +
        function () {}.constructor
    );
</script>

</body>
</html>

Preview

Undefined

A variable in JavaScript has the value undefined if it doesn't have a value. Furthermore, the type is unspecified. By changing the value to undefined, any variable may be cleared. Moreover, the kind will be unclear.

Example

<!DOCTYPE html>
<html>
<body>

<h1>JavaScript Undefined</h1>
<script>
    let Bike;
    document.write(typeof Bike);
</script>

</body>
</html>

Preview

Empty Values

Undefined has nothing to do with an empty value. An empty string has a type and a lawful value.

Example

<!DOCTYPE html>
<html>
<body>

<h1>JavaScript Empty Values</h1>
<script>
    let Bike = "";
    document.write(typeof Bike);
</script>

</body>
</html>

Preview

Null

Null is "nothing" in JavaScript. It is meant to represent something that does not exist. Regrettably, null's data type in JavaScript is an object. Setting an object to null and undefined will empty it.

Example

<!DOCTYPE html>
<html>
<body>

<h1>JavaScript Null</h1>
<script>
    let user = {fname:"Peter", lname:"Baker", age:26};
    user = null;
    document.write(typeof user);
</script>

</body>
</html>

Preview

Difference Between Undefined and Null

Null and undefined have the same value but are of distinct types.

Example

<!DOCTYPE html>
<html>
<body>

<h1>JavaScript Difference Between Undefined and Null</h1>
<script>
    document.write(
        typeof undefined + "<br>" +
        typeof null + "<br><br>" +
        (null === undefined) + "<br>" +
        (null == undefined)
    );
</script>

</body>
</html>

Preview

The instanceof Operator

If an object is an instance of the supplied object, the instanceof operator returns true.

Example

<!DOCTYPE html>
<html>
<body>

<h1>JavaScript instanceof Operator</h1>
<script>
    const lang = ["HTML", "CSS", "JavaScript", "jQuery"];

    document.write(
        (lang instanceof Array) + "<br>" +
        (lang instanceof Object) + "<br>" +
        (lang instanceof String) + "<br>" +
        (lang instanceof Number)
    );
</script>

</body>
</html>

Preview

The void Operator

After evaluating an expression, the void operator returns undefined. With "void(0)," this operator is frequently used to acquire the undefined primitive value (useful when evaluating an expression without using the return value).

Example

<!DOCTYPE html>
<html>
<body>

<h1>JavaScript void Operator</h1>
<a href="javascript:void(document.body.style.backgroundColor='green');">Click me to change the background color of body to green.</a>

</body>
</html>

Preview

JS Maps

JS Bitwise

JavaScript typeof

The typeof Operator

Example

Primitive Data

Example

Complex Data

Example

The Data Type of typeof

The constructor Property

Example

Undefined

Example

Empty Values

Example

Null

Example

Difference Between Undefined and Null

Example

The instanceof Operator

Example

The void Operator

Example

Links

Tutorials

Connect with us