Matching array in JavaScript

Question

I was playing with javaScript and found something confusing..

var a = [1,2,3];
var b = [1,2,3];
var c = '1,2,3';

a==b // false
b==c // true
c==a // true

What is happening behind the scenes ? anybody know that ?

Answer 1

Evidently arrays get cast to comma delimited strings when compared to strings. [UPDATE: as Sawant correctly states, it's calling the array's toString() method.] But arrays compared to arrays simply remain as arrays, and only the same instance will be equal to itself.

This is a good reason why you should always use === and never use ==.

If you need to compare the arrays by the elements they contain, use the isEqual function from the lodash library.

Answer 2

1. Comparing two objects (array), they are not the same!

   a==b // false
   

2. Comparing two strings, because an array compared to a string, will be casted to a string.

   b==c // true
   

3. The same as 2.

   c==a // true
   

When you compare with == the interpreter will try to make left-hand-side and right-hand-side the same type.

It will first format it to a string -> number -> boolean

So for example, comparing ![] == [].

1. ![] == []
2. false == []
3. false == ""
4. false == false

This are the steps the interpreter has to perform to come up with:

  ![] == [] // true

Because of all this, just use always the === operator, and it will be also much faster.

Answer 3

Array compared to another array is not the same because they are pointing at different memory addresses.

But an array compared to a string for equality (b == c), casts the array to a string; effectively, it's doing something similar to:

b.toString() === c

But with an identity comparison (===), it will also check the type of both variables, in which case b === c will render as false.