JavaScript typeof
In JavaScript there are 5 different data types that can contain values:
stringnumberbooleanobjectfunction
There are 6 types of objects:
ObjectDateArrayStringNumberBoolean
And 2 data types that cannot contain values:
nullundefined
The typeof Operator
You can use the typeof operator to find the data type of a
JavaScript variable.
Example
typeof "John"
// Returns "string"
typeof 3.14
// Returns "number"
typeof NaN
// Returns "number"
typeof false
// Returns "boolean"
typeof [1,2,3,4] // Returns
"object"
typeof {name:'John', age:34}
// Returns "object"
typeof new Date()
// Returns "object"
typeof function () {} // Returns
"function"
typeof myCar
// Returns "undefined" *
typeof null
// Returns "object"
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Please observe:
- The data type of NaN is number
- The data type of an array is object
- The data type of a date is object
- The data type of null is object
- The data type of an undefined variable is undefined *
- The data type of a variable that has not been assigned a value is also undefined *
You cannot use typeof to determine if a JavaScript object is an array (or a date).
Primitive Data
A primitive data value is a single simple data value with no additional properties and methods.
The typeof operator can return one of these primitive types:
stringnumberbooleanundefined
Example
typeof "John" // Returns
"string"
typeof 3.14 // Returns
"number"
typeof true // Returns
"boolean"
typeof false // Returns
"boolean"
typeof x
// Returns "undefined" (if x has no value)
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Complex Data
The typeof operator can return one of two complex types:
functionobject
The typeof operator returns "object" for objects, arrays, and null.
The typeof operator does not return "object" for functions.
Example
typeof {name:'John', age:34} // Returns "object"
typeof [1,2,3,4]
// Returns "object" (not "array", see note below)
typeof null // Returns
"object"
typeof function myFunc(){} // Returns "function"
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The typeof operator returns "object" for arrays because in JavaScript arrays are objects.
The Data Type of typeof
The typeofoperator is not a variable. It is an operator. Operators ( + - * /
) do not have any data type.
But, the typeof operator always returns a string (containing
the type of the operand).
The constructor Property
The constructor property returns the constructor
function for all JavaScript variables.
Example
"John".constructor
// Returns function String() {[native code]}
(3.14).constructor
// Returns function Number() {[native code]}
false.constructor // Returns
function Boolean() {[native code]}
[1,2,3,4].constructor
// Returns function Array() {[native code]}
{name:'John',age:34}.constructor
// Returns function Object() {[native code]}
new Date().constructor
// Returns function Date() {[native code]}
function () {}.constructor // Returns
function Function(){[native code]}
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You can check the constructor property to find out if an object is an Array
(contains the word "Array"):
Example
function isArray(myArray) {
return myArray.constructor.toString().indexOf("Array") > -1;
}
Or even simpler, you can check if the object is an Array function:
You can check the constructor property to find out if an object is a
Date (contains the word "Date"):
Example
function isDate(myDate) {
return myDate.constructor.toString().indexOf("Date") > -1;
}
Or even simpler, you can check if the object is a Date function:
Undefined
In JavaScript, a variable without a value, has the value undefined.
The type is also undefined.
Any variable can be emptied, by setting the value to undefined.
The type will also be undefined.
Empty Values
An empty value has nothing to do with undefined.
An empty string has both a legal value and a type.
Null
In JavaScript null is "nothing". It is supposed to be something that doesn't exist.
Unfortunately, in JavaScript, the data type of null is an object.
You can consider it a bug in JavaScript that typeof null is an object. It should be null.
You can empty an object by setting it to null:
Example
let person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"};
person = null; //
Now value is null,
but type is still an object
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You can also empty an object by setting it to undefined:
Example
let person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"};
person = undefined; //
Now both value and type is undefined
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Difference Between Undefined and Null
undefined and null are equal in value but different in type:
typeof undefined
// undefined
typeof null
// object
null === undefined
// false
null == undefined
// true
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The instanceof Operator
The instanceof operator returns true if an object is an instance of the specified object:
Example
const cars = ["Saab", "Volvo", "BMW"];
(cars instanceof Array);
(cars instanceof Object);
(cars instanceof String);
(cars instanceof Number);
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The void Operator
The void operator evaluates an expression and returns undefined. This operator is often used to obtain the undefined primitive value, using "void(0)" (useful when evaluating an expression without using the return value).
Example
<a href="javascript:void(0);">
Useless link
</a>
<a href="javascript:void(document.body.style.backgroundColor='red');">
Click me to change the background color of body to red
</a>
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