Learn JS #30

JavaScript typeof


In JavaScript there are 5 different data types that can contain values:

  • string
  • number
  • boolean
  • object
  • function

There are 6 types of objects:

  • Object
  • Date
  • Array
  • String
  • Number
  • Boolean

And 2 data types that cannot contain values:

  • null
  • undefined

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"

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:

  • string
  • number
  • boolean
  • undefined

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)

Complex Data

The typeof operator can return one of two complex types:

  • function
  • object

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"

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]}

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:

Example

function isArray(myArray) {
  return myArray.constructor === Array;
}

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:

Example

function isDate(myDate) {
  return myDate.constructor === Date;
}

Undefined

In JavaScript, a variable without a value, has the value undefined. The type is also undefined.

Example

let car;    // Value is undefined, type is undefined

Any variable can be emptied, by setting the value to undefined. The type will also be undefined.

Example

car = undefined;    // Value is undefined, type is undefined

Empty Values

An empty value has nothing to do with undefined.

An empty string has both a legal value and a type.

Example

let car = "";    // The value is "", the typeof is "string"

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

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

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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