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:

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…

Answer 1

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

1. 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).

2. var crazy = {}+[];
typeof crazy //"string"
   In this case you are adding object and array – they both coerce to string, therefore typeof returns “string”.

3. {}+[]
   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.