JavaScript Array behaving like an object - Stack Overflow

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I am trying to understand something in JavaScript.

Lets say I do the following:

   let x = [];
   x['Monday'] = 'Work';
   x['Tuesday'] = 'More Work';

   console.log(x) //[Monday: "Work", Tuesday: "More Work"]
   console.log(x.Monday) //"Work"
   console.log(x.Tuesday) //"More Work"

Can someone help explain why the array now behaves like an Object?

I am trying to understand something in JavaScript.

Lets say I do the following:

   let x = [];
   x['Monday'] = 'Work';
   x['Tuesday'] = 'More Work';

   console.log(x) //[Monday: "Work", Tuesday: "More Work"]
   console.log(x.Monday) //"Work"
   console.log(x.Tuesday) //"More Work"

Can someone help explain why the array now behaves like an Object?

• javascript
• arrays
• object

Share Improve this question Follow asked Jan 18, 2018 at 7:39 user5844628user5844628 41911 gold badge66 silver badges1717 bronze badges 4

• 2 You've created an associative array where you have keys and values. x.Monday is not an entry with the value Monday, you're setting the key Monday to have the value of Work. Also, arrays extend Object, in-fact most things in Javascript extend object so theres that too – Halfpint Commented Jan 18, 2018 at 7:41
• 1 You'd be amazed how far you could take the bracket notation versus the dot notation: developer.mozilla/nl/docs/Web/JavaScript/Reference/… – rolfv1 Commented Jan 18, 2018 at 7:42
• A = { b : "hi"} looks the same as A['b'] = "hi"; because they both have keys to access value the only difference is that of the array is associative and the system knows which datatype you are referring to, normally an object and an array are similar just with certain things distinguishing them – Ezekiel Commented Jan 18, 2018 at 7:44
• Possible duplicate of Are Javascript arrays primitives? Strings? Objects? – Mohammad Usman Commented Jan 18, 2018 at 8:00

Add a ment  | 

5 Answers 5

Sorted by: Reset to default 8

Because array IS an Object

[] instanceof Object
> true

[] instanceof Array
> true

Both give you true because Array extends normal Object functionality. Furthermore, the array indexes all its members by string values

const array = [];

array[0] = 1;

array['0'] // will give you 1

which is also unexpected by most of the people. So, the array behaves like an Object even if you use it as a normal array with indices

Everything in JS is an Object except the Primitive types (and even those e with Object wrappers)

That is why you can access properties and methods like array.length array.indexOf() etc just like you would with any other object. The indices are like special properties on the array object which are iterable

over all JavaScript array is also an object

if you check the type of x

typeof x

it will prints "object"

var x =[];
x.name = "X";

here we are creating a property "name" of x array

Because in javascript there is no real Array type as a low level type. Array class is just a kind of a class which extended from Object class.

• Every class in javascript except primitives like Number, String ect. are extended from Object class.
• Every object (instance of class) can have properties and methods.
• Every instance of classes can have new properties and methods on the run-time as well as this properties could be changed modified or deleted.

So by doing this.

var ar = [] // I used array literal. Not totally same but similar of doing new Array();
arr['foo'] = 'bar';

You are just adding a new property to an object.

I think you’ve heard by now that Array is a type of Object, so I won’t beat that dead horse any further. Well maybe a little.

Which means you can accsss and set properties with names like “Monday” and “Tuesday”. Since it’s a type of Object this is no problem (and a statement like a[‘1’] is implicitly converting that string ‘1’ to a number 1). But don’t mistake this property name to be an array index, those are specifically integers**. What’s the importance of that?

Arrays have a nice behaviour that when you use an actual array index, the length property is automatically updated for you. Not to mention array index access is usually optimized by JVM implementations to be noticeably faster than regular property accesses.

Also let’s not forget one of the other huge benefits of having Arrays, they inherit from the Array.prototype, which has a whole lot of useful methods :)!

So in short use Arrays like Arrays — gain the perf boost while also making sure the code is easier to prehend.

Source: JS Definitive Guide 6th - Array chapter

(** non-negative and less than 2^32-2 since arrays has 32-bit size indexes.)

I am trying to understand something in JavaScript.

Lets say I do the following:

   let x = [];
   x['Monday'] = 'Work';
   x['Tuesday'] = 'More Work';

   console.log(x) //[Monday: "Work", Tuesday: "More Work"]
   console.log(x.Monday) //"Work"
   console.log(x.Tuesday) //"More Work"

Can someone help explain why the array now behaves like an Object?

I am trying to understand something in JavaScript.

Lets say I do the following:

   let x = [];
   x['Monday'] = 'Work';
   x['Tuesday'] = 'More Work';

   console.log(x) //[Monday: "Work", Tuesday: "More Work"]
   console.log(x.Monday) //"Work"
   console.log(x.Tuesday) //"More Work"

Can someone help explain why the array now behaves like an Object?

• javascript
• arrays
• object

Share Improve this question Follow asked Jan 18, 2018 at 7:39 user5844628user5844628 41911 gold badge66 silver badges1717 bronze badges 4

• 2 You've created an associative array where you have keys and values. x.Monday is not an entry with the value Monday, you're setting the key Monday to have the value of Work. Also, arrays extend Object, in-fact most things in Javascript extend object so theres that too – Halfpint Commented Jan 18, 2018 at 7:41
• 1 You'd be amazed how far you could take the bracket notation versus the dot notation: developer.mozilla/nl/docs/Web/JavaScript/Reference/… – rolfv1 Commented Jan 18, 2018 at 7:42
• A = { b : "hi"} looks the same as A['b'] = "hi"; because they both have keys to access value the only difference is that of the array is associative and the system knows which datatype you are referring to, normally an object and an array are similar just with certain things distinguishing them – Ezekiel Commented Jan 18, 2018 at 7:44
• Possible duplicate of Are Javascript arrays primitives? Strings? Objects? – Mohammad Usman Commented Jan 18, 2018 at 8:00

Add a ment  | 

5 Answers 5

Sorted by: Reset to default 8

Because array IS an Object

[] instanceof Object
> true

[] instanceof Array
> true

Both give you true because Array extends normal Object functionality. Furthermore, the array indexes all its members by string values

const array = [];

array[0] = 1;

array['0'] // will give you 1

which is also unexpected by most of the people. So, the array behaves like an Object even if you use it as a normal array with indices

Everything in JS is an Object except the Primitive types (and even those e with Object wrappers)

That is why you can access properties and methods like array.length array.indexOf() etc just like you would with any other object. The indices are like special properties on the array object which are iterable

over all JavaScript array is also an object

if you check the type of x

typeof x

it will prints "object"

var x =[];
x.name = "X";

here we are creating a property "name" of x array

Because in javascript there is no real Array type as a low level type. Array class is just a kind of a class which extended from Object class.

• Every class in javascript except primitives like Number, String ect. are extended from Object class.
• Every object (instance of class) can have properties and methods.
• Every instance of classes can have new properties and methods on the run-time as well as this properties could be changed modified or deleted.

So by doing this.

var ar = [] // I used array literal. Not totally same but similar of doing new Array();
arr['foo'] = 'bar';

You are just adding a new property to an object.

I think you’ve heard by now that Array is a type of Object, so I won’t beat that dead horse any further. Well maybe a little.

Which means you can accsss and set properties with names like “Monday” and “Tuesday”. Since it’s a type of Object this is no problem (and a statement like a[‘1’] is implicitly converting that string ‘1’ to a number 1). But don’t mistake this property name to be an array index, those are specifically integers**. What’s the importance of that?

Arrays have a nice behaviour that when you use an actual array index, the length property is automatically updated for you. Not to mention array index access is usually optimized by JVM implementations to be noticeably faster than regular property accesses.

Also let’s not forget one of the other huge benefits of having Arrays, they inherit from the Array.prototype, which has a whole lot of useful methods :)!

So in short use Arrays like Arrays — gain the perf boost while also making sure the code is easier to prehend.

Source: JS Definitive Guide 6th - Array chapter

(** non-negative and less than 2^32-2 since arrays has 32-bit size indexes.)

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