javascript Unary plus between brackets

Question

1.{}+[] gives 0

2.[]+{} gives [object object] I know how 1 and 2 comes but

3.{}+[]+{} gives [object object][object object]

how shouldn't it be 0[object object]?

@mplungjan: I think these are all wrong. https://stackoverflow.com/q/36438034/218196 is the right duplicate. — Felix Kling, Mar 25 '19 at 07:24
Chrome uses a simple heuristic and evaluates a line that starts with `{` and ends with `}` as an expression. Therefore both `{}` in `{}+[]+{}` are evaluated as objects an object, whereas the `{}` in `{}+[]` is a block. — Felix Kling, Mar 25 '19 at 07:25
See https://stackoverflow.com/questions/36438034/why-is-no-longer-nan-in-chrome-console . The expression is slightly different, but the reason/answer is the same. — Felix Kling, Mar 25 '19 at 07:26
Well done @FelixKling - It was a dupe of something and the rest are relevant to show type coercion — mplungjan, Mar 25 '19 at 07:31
To be absolutely clear: You will get `0[object object]` in other environments (e.g. Firefox). It's just the Chrome console that interprets `{}+[]+{}` as if you write `({}+[]+{})`. — Felix Kling, Mar 25 '19 at 07:34
Other related questions: https://stackoverflow.com/q/53712304/218196, https://stackoverflow.com/q/50989247/218196 — Felix Kling, Mar 25 '19 at 07:37

Jack Bashford · Answer 1 · 2019-03-25T07:34:54.143

0

When you use console.log, it executes toString - which when using {} + [], gives [object Object].

console.log({} + []);

(Note: simply placing {} + [] into the Developer Tools console gives 0, because toString is not called).

That's why this yields [object Object][object Object]:

console.log({} + [] + {});

Because you're adding an object (formed from {} + []) to another object, and because you're using the + concatenation operator, this forces toString - therefore, you're adding [objectObject] and [object Object], yielding [object Object][object Object].

Note: The above all are on Chrome. In Firefox and potentially others, they work differently, which is why some things in your question are different.

edited Mar 25 '19 at 07:34

answered Mar 25 '19 at 07:19

Jack Bashford

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`({}+[])` this will `[object object]` why – Sourabh Somani Mar 25 '19 at 07:21
what is the difference between `({}+[])` vs `{}+[]` vs `console.log({}+[])` – Sourabh Somani Mar 25 '19 at 07:22
Because in `console.log`, it calls `toString`. – Jack Bashford Mar 25 '19 at 07:23
ok but what about `({}+[])` it is also calling `toString()` – Sourabh Somani Mar 25 '19 at 07:24
That's because of the `()` – Jack Bashford Mar 25 '19 at 07:25
Writing in Chrome console without `console.log` gives you a different answer because it's not calling `toString`. `console.log` calls `toString`, which on an object yields `[object Object]`. – Jack Bashford Mar 25 '19 at 07:25
Since you are already saying that `console.log` is different than having `{} + []` on its own, how can you say that their information is wrong? They are simply executing their code differently. – Felix Kling Mar 25 '19 at 07:28
I'm sorry @FelixKling, I know what you're saying, I just don't know how to phrase it correctly - how should I do so? – Jack Bashford Mar 25 '19 at 07:29
@FelixKling I edited my answer, is it better now? – Jack Bashford Mar 25 '19 at 07:30
While you are explaining what `{} + [] + {}` results in when evaluated as expression, the missing piece is why `{} + [] + {}` is evaluated as expression in the first place. In Firefox the same code yields `"0[object Object]"`, which shows that is evaluated differently (and correctly btw). – Felix Kling Mar 25 '19 at 07:31
I don't know why that is - it works in Firefox (just tested myself) but works differently in Chrome (gives `[object Object][object Object]`). I'll put that in my answer. – Jack Bashford Mar 25 '19 at 07:32
Well, that's what the question is about (kind of) ;) The answer is in the duplicate. – Felix Kling Mar 25 '19 at 07:33

javascript Unary plus between brackets

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