Why does ++[[]][+[]]+[+[]] return the string “10”?

Question

This is valid and returns the string \"10\" in JavaScript (more examples here):

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Answer 1

This one evaluates to the same but a bit smaller

+!![]+''+(+[])

• [] - is an array is converted that is converted to 0 when you add or subtract from it, so hence +[] = 0
• ![] - evaluates to false, so hence !![] evaluates to true
• +!![] - converts the true to a numeric value that evaluates to true, so in this case 1
• +'' - appends an empty string to the expression causing the number to be converted to string
• +[] - evaluates to 0

so is evaluates to

+(true) + '' + (0)
1 + '' + 0
"10"

So now you got that, try this one:

_=$=+[],++_+''+$

Answer 2

++[[]][+[]] => 1 // [+[]] = [0], ++0 = 1
[+[]] => [0]

Then we have a string concatenation

1+[0].toString() = 10

Answer 3

1. Unary plus given string converts to number
2. Increment operator given string converts and increments by 1
3. [] == ''. Empty String

4. +'' or +[] evaluates 0.

   ++[[]][+[]]+[+[]] = 10 
   ++[''][0] + [0] : First part is gives zeroth element of the array which is empty string 
   1+0 
   10