为什么{} == {}为false，但{} + {} == {} + {}为真

Question

Why in node.js is {} == {} equivalent to false , but is {} + {} == {} + {} equivalent to true ?

> {} == {}
false

> {} + {} == {} + {}
true

Answer 1

+ here is the string-concatenation operator. This:

{} == {}

means "if I create one object with {} , and another object with {} , are they the same object?"; and the answer is "no".

This:

{} + {} == {} + {}

means "is the primitive string "[object Object][object Object]" the same as the primitive string "[object Object][object Object]" ?"; and the answer is "yes".

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Edited to add: A number of commenters point out that in Chrome's Web Console, {} + {} performs numeric addition, NaN + NaN , such that {} + {} == {} + {} actually returns false (because it's not true that NaN == NaN ). Firefox's Web Console gives the same result as Chrome's, but if you run this inside a page, it gives the same result as node.js.

[Redacted: long explanation of how the spec dictates that {} + {} should be string concatenation and {} + {} == {} + {} should be true; the explanation, while correct, is no longer terribly interesting, given the below.]

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Edited to add: Thanks to a comment by jJ' , I can now offer a better explanation of the inconsistency.

The reason for the Web Console's behavior is that the Web Console does not specifically require an expression; it will happily accept something like if(true) { } . So, when it sees {} + {} , it doesn't interpret that as an expression; the initial {} is interpreted as a naked block, then the + {} is interpreted as an expression (creating a new object, converting it to a primitive number — namely NaN — and evaluating to that number). The Web Console displays the result of the last expression (eg, the input 3; 4 will give the output 4 ), which in this case is NaN .

{} + {} == {} + {} , similarly, is interpreted as "empty block, followed by (+{}) == ({} + {}) ", ie, "empty block, followed by NaN == '[object Object][object Object]' ", ie "empty block, followed by false ".

This can be fixed by using parentheses; ({} + {} == {} + {}) , for example, returns true .

(This behavior, by the way, is not completely specific to the Web Console. eval follows the same rules, such that eval('{} + {}') is NaN and eval('({} + {})') is '[object Object][object Object]' .)