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The typeof operator returns the data type of a JavaScript variable.
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.
Examples typeof "John" // Returns string typeof ("John"+"Doe") // Returns string typeof 3.14 // Returns number typeof 33
Formula
// 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 undefined typeof null // Returns object In JavaScript, null is a primitive value. However, typeof returns object.
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This is a well - known bug in JavaScript and has historical reasons.Formula
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
Example 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.
How to know if a variable is an array? ECMAScript 5 (2009) defined a new method for this: Array.isArray()
// Create an Array const fruits = ["apples", "bananas", "oranges"];
Array.isArray(fruits);The instanceof operator returns true if an object is an instance of a specified object type:
// Create a Date const time = new Date();
(time instanceof Date);
// Create an Array const fruits = ["apples", "bananas", "oranges"];
(fruits instanceof Array);
// Create a Map const fruits = new Map([
["apples", 500],
["bananas", 300],
["oranges", 200]
]);
(fruits instanceof Map);
// Create a Set const fruits = new Set(["apples", "bananas", "oranges"]);
(fruits instanceof Set);The typeof of an undefined variable is undefined.
Example typeof car;The typeof of a variable with no value is undefined. The value is also undefined.
let car;
typeof car;Any variable can be emptied, by setting the value to undefined. The type will also be undefined.
let car = "Volvo";
car = undefined;An empty value has nothing to do with undefined. An empty string has both a legal value and a type.
let car = "";
typeof car;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
// Create an Object 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
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
The constructor property returns the constructor function for all JavaScript variables.
// 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
(myArray.constructor === Array);With the constructor, you can check if an object is a
(myDate.constructor === Date);
All Together typeof "John"
// Returns "string"
typeof ("John"+"Doe")
// Returns "string"typeof 3.14 // Returns "number"
Formula
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"
(Not a Number) is number !
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).