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 array is object
- The data type of date is object
- The data type of null is object
- The data type of undefined variable is undefined *
- The data type of 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 typeof operator 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;
}
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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.
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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