Flash cards
Review the key moves
What is the main idea behind JavaScript typeof?
Lesson checks
Practice each idea before moving on
Short Mimo-style checks built from this lesson's code, terms, and sequence.
Which statement best captures the main point of this lesson?
Complete the missing token from the example code.
___ "John"Put the learning moves in the order that makes the concept easiest to apply.
The typeof Operator
The typeof operator returns the data type of a JavaScript variable.
Primitive Data Types
In JavaScript, a primitive value is a single value with no properties or methods.
JavaScript has 7 primitive data types
- string
- number
- boolean
- bigint
- symbol
- null
- undefined
The typeof operator returns the type of a variable or an expression.
typeof "John"
// Returns string
typeof ("John"+"Doe")
// Returns string
typeof 3.14
// Returns number
typeof 33
// Returns number
typeof (33 + 66)
// Returns number
typeof true
// Returns boolean
typeof false
// Returns boolean
typeof 1234n
// Returns bigint
typeof Symbol()
// Returns symbol
typeof x
// Returns undefinedIn JavaScript, null is a primitive value.
However, typeof returns object.
This is a well-known bug in JavaScript and has historical reasons.
Complex Data Types
A complex data type can store multiple values and/or different data types together.
JavaScript has one complex data type
- object
All other complex types like arrays, functions, sets, and maps are just different types of objects.
The typeof operator returns only two types:
- object
- function
typeof {name:'John'} // Returns object
typeof [1,2,3,4]
// Returns object
typeof new Map()
// Returns object
typeof new Set()
// Returns object
typeof function (){} // Returns functionThe typeof operator returns object for all types of objects: objects arrays dates sets maps You cannot use typeof to determine if a JavaScript object is an array or a date.
- objects
- arrays
- dates
- sets
- maps
You cannot use typeof to determine if a JavaScript object is an array or a date.
How to Recognize an Array
How to know if a variable is an array?
ECMAScript 5 (2009) defined a new method for this: Array.isArray() :
Example
// Create an Array
const fruits = ["apples", "bananas", "oranges"];
Array.isArray(fruits);The instanceof Operator
The instanceof operator returns true if an object is an instance of a specified object type:
Examples
// Create a Date
const time = new Date();
(time instanceof Date);Undefined Variables
typeof car;The typeof of a variable with no value is undefined . The value is also undefined .
Example
let car;
typeof car;Any variable can be emptied, by setting the value to undefined .
The type will also be undefined .
Example
let car = "Volvo";
car = 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 = "";
typeof car;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 empty an object by setting it to null :
Example
// Create an Object
let person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"};
person = null;
// Now value is null, but type is still an objectYou 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 undefinedDifference 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 // trueThe constructor Property
The constructor property returns the constructor function for all JavaScript variables.
Example
// Returns function Object() {[native code]}: {name:'John',age:34}.constructor // Returns function Array() {[native code]}: [1,2,3,4].constructor // Returns function Date() {[native code]}: new Date().constructor // Returns function Set() {[native code]}: new Set().constructor // Returns function Map() {[native code]}: new Map().constructor // Returns function Function() {[native code]}: function () {}.constructorWith the constructor, you can check if an object is an Array :
(myArray.constructor === Array);With the constructor, you can check if an object is a Date :
(myDate.constructor === Date);typeof "John"
// Returns "string"
typeof ("John"+"Doe")
// Returns "string"
typeof 3.14
// Returns "number"
typeof (33 + 66)
// Returns "number"
typeof NaN
// Returns "number"
typeof 1234n
// Returns "bigint"
typeof true
// Returns "boolean"
typeof false
// Returns "boolean"
typeof {name:'John'}
// Returns "object"
typeof [1,2,3,4]
// Returns "object"
typeof {}
// Returns "object"
typeof []
// Returns "object"
typeof new Object()
// Returns "object"
typeof new Array()
// Returns "object"
typeof new Date()
// Returns "object"
typeof new Set()
// Returns "object"
typeof new Map()
// Returns "object"
typeof function () {}
// Returns "function"
typeof x
// Returns "undefined"
typeof null
// Returns "object"The data type of NaN (Not a Number) is number !
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>