7.3 JS Typeof

The typeof Operator

You can use the typeof operator to find the data type of a JavaScript variable.

Example 1: JS typeof Operator part 1

 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 2: JS typeof Operator part 2

 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 3: JS typeof Operator part 3

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 constructor Property

The constructor property returns the constructor function for all JavaScript variables.

Example 4: JS typeof Properties - constructor Property

 "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 5: JS typeof Properties - constructor Property part 2

function isArray(myArray) {
  return myArray.constructor.toString().indexOf("Array") > -1;
}

Or even simpler, you can check if the object is an Array function:

Example 6: JS typeof Properties - Array Object

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 7: JS typeof Operator - Date Object

function isDate(myDate) {
  return myDate.constructor.toString().indexOf("Date") > -1;
}

Or even simpler, you can check if the object is a Date function:

Example 8: JS typeof Operator - Date Function

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 9: JS typeof Operator - undefined part 1

 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 10: JS typeof Operator - undefined part 2

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 11: JS typeof Operator - empty values

 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 12: JS typeof Operator - null

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 13: JS typeof Operator - undefined Data Type

let person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"};
person = undefined;   // Now both value and type is undefined

The instanceof Operator

The instanceof operator returns true if an object is an instance of the specified object:

Example 14: JS instanceof Operator

const cars = ["Saab", "Volvo", "BMW"];

(cars instanceof Array);
(cars instanceof Object);
(cars instanceof String);
(cars instanceof 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 15: JS void Operator

<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>