Why does this happen?

Question

Running this code produces results as in comments, but why is it happening?

let a = [1,2,3];
let b = [1,2,3];
let c = '1,2,3';
a == c; // true
b == c; // true
a == b; // false

I noticed if you change the previous code with the following, final result changes:

let a = [1, 2, 3];
let b = [1, 2, 3];
let c = '1, 2, 3';
a == c; // false
b == c; // false
a == b; // false

Thank you!

Because `a==c` and `b==c` convert the arrays to strings, the third compares references of the two arrays (which are not equal). See [here](https://stackoverflow.com/questions/7625144/implicit-data-type-conversion-in-javascript-when-comparing-integer-with-string-u) — Federico klez Culloca, May 04 '18 at 15:32
Forced same type. when comparing primitive elements like strings, numbers or booleans to complex elements like objects, functions or arrays, the latter will be cast to the nearest primitive (string). When comparing complex elements, you are actually checking if the two sides are references to the same elements (instead of whether the objects looks the same). — Emil S. Jørgensen, May 04 '18 at 15:35
Are you asking why `[1,2,3] == '1,2,3'`, or why `[1,2,3] != [1,2,3]`? That's two separate questions. — Bergi, May 04 '18 at 15:42
@Bergi two separate questions, both of which are answered by "don't use broken language features" ;) — Patrick Roberts, May 04 '18 at 16:11
@PatrickRoberts I was thinking of "…which are answered by separate duplicates" :-) Mostly https://stackoverflow.com/q/7314635/1048572 — Bergi, May 04 '18 at 16:23

31piy · Answer 1 · 2018-05-04T16:00:58.290

3

Please see this article which describes the different types of equality checks in JavaScript.

a == c; // true
b == c; // true

Both of the above return true because == causes the array to convert to its string equivalent using its toString method (as it is being compared to a string using == operator), which returns "1,2,3". Clearly "1,2,3" is equal to "1,2,3".

a == b; // false

The above returns false because a and b are two different arrays and JavaScript compares them by references instead of traversing each element and comparing those individually or by converting them to equivalent strings.

edited May 04 '18 at 16:00

answered May 04 '18 at 15:33

31piy

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why does the second example behave different? thanks! – blurstream May 04 '18 at 15:42
ye still 1st example [1,2,3].toString()==[1,2,3].toString() = true and a == b gives false why – vivid May 04 '18 at 15:43
@vivid because there's no conversion when both operands are of the same type, so there's no `.toString()` involved, just object reference comparison. – Federico klez Culloca May 04 '18 at 15:52
@FedericoklezCulloca but == should check value not references :O – vivid May 04 '18 at 15:56
1

@vivid, not in the case of objects and arrays. – Federico klez Culloca May 04 '18 at 15:59
1

@vivid -- `toString()` converts the array to a string and `==` happens on two strings in `[1,2,3].toString()==[1,2,3].toString()`, which are compared by values, and thus, it returns `true`. – 31piy May 04 '18 at 15:59
@FedericoklezCulloca then person that "made concept of ==" was probably best troll of internet ;p – vivid May 04 '18 at 16:01
@31piy sure but u asking to check value not refs;/ == should check only value and === should check value and type isnt that true? maybe i dont know == operator-.- – vivid May 04 '18 at 16:05
@vivid -- please see the link which I posted in my answer. – 31piy May 04 '18 at 16:37

score 1 · Answer 2 · answered May 04 '18 at 16:05

@blurstream The thing is that the implicit conversion from array to string changes the test from "[1,2,3]" == "[1,2,3]" without the spaces to "[1,2,3]" == "[1, 2, 3]" with the spaces. That's why it changes.

The array [1, 2, 3] is understood by JavaScript as [1,2,3] without the spaces. That changes the test!

More specifically, that changes from "[1,2,3]" == "[1,2,3]", which returns true, to "[1,2,3]" == "[1, 2, 3]", which returns false.

P.S: I didn't comment back and instead posted an answer because of my lack of reputation here, please forgive me.

Why does this happen?

2 Answers2