Question

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What is the explanation for these bizarre JavaScript behaviours mentioned in the 'Wat' talk for CodeMash 2012?

When I type

{} + []

in the Google Chrome JavaScript console, I get

0

as a result. However, when I type

Function("return {} + []")()

I get

"[object Object]"

as a result. I would think that both operations should return the same result, as one is simply a wrapper around the other. Why do they return different results?

Answer 1

The core reason is that {} means a different thing in a statement context { statement0; statement1 } than in an expression context ({ "property": value, ... }).

 {} + []

is a block and a unary comparison operator so the same as

{}  // An empty block of statements.
(+ [])  // Use of prefix operator +.

The other is a use of the plus operator which when used with two objects concatenates them as in

return String({}) + String([])

Since Array.prototype.toString joins the array on commas, it is similar to

return String({}) + [].join(",")

which reduces to

return "[Object object]" + "";

and finally to

return "[Object object]"