Why show object object when input ({} + {}) in Javascript?

Question

Why this happened

{}+{} output- NaN

({} + {})output- ''[object Object][object Object]''

Why show that kind of result?

see also [Why {} + {} is NaN only on the client side? Why not in Node.js?](http://stackoverflow.com/q/17268468/1048572) — Bergi, Aug 02 '15 at 18:09
…and [Why plus operator in brackets works differently?](http://stackoverflow.com/q/25416725/1048572) — Bergi, Aug 02 '15 at 18:15

Domino · Accepted Answer · 2019-02-11T15:45:48.667

0

When you write {} in some ways, if what the "duplicate" answer says is correct, it might be interpreted as a code block instead of an empty object. The empty block has no return statement, so it returns undefined.

undefined + undefined returns NaN because it tries to do a mathematical addition between two values that are not numbers. When you write {} + {} but don't do anything with the result, Chrome again evaluates the + as the addition and both {} as undefined.

However, as soon as you actually do something with the result, the behavior is different because the browser understands that you're using the implicit object declaration syntax. If you do say, var x = {} + {}, Chrome creates two objects and tries to convert them into a value that can be used with the + operator. So it converts the objects into string representations and concatenates the strings together.

The same thing happens when you use parentheses, because blocks can't exist inside parenthesis like that, so it can't mistake the {} for an empty block.

PS: I had it wrong at first and I'm just taking what I read on that other answer now.

edited Feb 11 '19 at 15:45

answered Aug 02 '15 at 17:48

Domino

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No, `{}` is not evaluated to `undefined`. There is absolutely no `undefined` involved in any of the expressions. – Bergi Aug 02 '15 at 18:11
On IE this {}+{} will cause blue screen of death – Medet Tleukabiluly Aug 02 '15 at 18:12
@evc: Which IE version? – Bergi Aug 02 '15 at 18:12
@Bergi I know that it makes no sense that is does in terms of the JS standard, but just type `{}` in Chrome's console and it returns undefined. – Domino Aug 03 '15 at 21:45
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Oh, I'll edit my answer to point out what the "dupplicate" said. – Domino Aug 03 '15 at 21:48
@JacqueGoupil: Because that's an empty block, and does nothing. For the `undefined`, see [Chrome/Firefox console.log always appends a line saying undefined](http://stackoverflow.com/q/14633968/1048572) – Bergi Aug 03 '15 at 21:50
Thanks. Can you also add why `{} +{}` leads to `NaN`? – Bergi Aug 03 '15 at 22:19

Why show object object when input ({} + {}) in Javascript?

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