Problems with adding object with array

Question

Inspired by this video, I tested further with {}+[].

Test 1:

typeof {}+[]  //"object"

Okay, so {}+[] is an object.

Test 2:

var crazy = {}+[];
typeof crazy  //"string"

What? Didn't {}+[] is an object? Why is it a string now?

Test 3:

console.log({}+[])

What I got:

enter image description here

So it is a number!... No?

So what actually is the type of {}+[]??

UPDATED

To people who say {}+[] is a empty string:

{}+[] === ""     //false
({}+[]) === ""   //false
({};+[]) === ""  //SyntaxError
({}+[]).length   //15

JavaScript is so hard to understand...

I think in test1, typeof finds first paramater's type -> {}, and it's an object, try typeof ({}+[]) this one, it's string. — Okan Kocyigit
– Okan Kocyigit, Commented Apr 29, 2012 at 22:20
@pcamal, So is it a number or a string? (Thought {}+[] would be object, because both of them are.) — Derek 朕會功夫
– Derek 朕會功夫, Commented Apr 29, 2012 at 22:22
Regarding your update: {}+[] === "" is evaluated as {}; +[] === "";, i.e. empty block and +[] === "". {}+[] === 0 yields true. — Felix Kling
– Felix Kling, Commented Apr 29, 2012 at 22:29
JavaScript is so hard to understand: Well, that's because you are ambiguous ;) When encountering {} in {}+[] === "" the parser does not know whether {} should indicate an object literal or a block. Since this is not in an expression context, {} is interpreted as block (the default behavior). The parenthesis (...) force an evaluation as expression. — Felix Kling
– Felix Kling, Commented Apr 29, 2012 at 22:34
I just added this to indicate that the line {}+[] is interpreted as two statements and not as one. But since JavaScript has automatic semicolon insertion it might actually do this. Why? Because if some syntax is ambiguous, a decision has to be made how to interpret it. In this case, the developers decided to interpret {}+[] as block, unary plus, array and not as object, addition operator, array. You might not agree with this, but that's how it is :) — Felix Kling
– Felix Kling, Commented Apr 29, 2012 at 22:50

Li0liQ · Accepted Answer · 2012-04-29 22:43:27Z

13

Type of {}+[] may vary depending on the context.

typeof {}+[] //"object"
As per operators precedence in this case typeof {} evaluates to "object", +[] adds an empty string(array is coerced to string) therefore result is "object".
You could think of checking typeof ({}+[]) (your second case).
var crazy = {}+[]; typeof crazy //"string"
In this case you are adding object and array - they both coerce to string, therefore typeof returns "string".
{}+[]
This is interpreted as an empty block of code, unary plus and empty array. First part does nothing, array is converted to a comma-separated string of it's elements(empty string for empty array), then to a number(empty string is converted to 0), hence 0.

UPDATED

{}+[] === "" //false
see #3, {} is interpreted as a block, you are getting 0 on the left.
Compare {}+[] === 0 // true.
({}+[]) === "" //false
see #1, {} is interpreted as an object literal. When trying to add array and object, they both convert to string, "[object Object]" for object and empty string for array. Hence, you are getting "[object Object]" on the left.
Compare ({}+[]) === "[object Object]" // true.
({};+[]) === "" //SyntaxError
I guess, this one is self-explanatory :)
({}+[]).length //15
15 is exactly the length of "[object Object]", see above.

edited Apr 29, 2012 at 22:43

answered Apr 29, 2012 at 22:22

Li0liQ

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3 Comments

Felix Kling Over a year ago

@Derek: Because +"" is 0. @Li0liQ: Btw, the string is not converted to a number equal to the length of the string. +"abc" returns NaN. How the unary plus converts strings is defined in the specification.

VisioN Over a year ago

Yep, and ({}+[]) in the console will give you object.

Li0liQ Over a year ago

@Felix Kling Thanks for the remark, indeed, array is converted to a comma-separated list of it's values or empty string in case of empty array. Empty string is converted to 0.

Yaron U. · Accepted Answer · 2012-04-29 22:40:15Z

0

var f = {}+[];
console.log(f==="[object Object]")

this will log true because when you assign {}+[] to the var it converts it to string by using the toString() method of it... which in this case returns "[object Object]" (which is 15 letters long - and that where the length=15 is from)

answered Apr 29, 2012 at 22:40

Yaron U.

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