Javascript Compiler behavior - double plus for array of empty array and array of zero is.. ONE
javascript
Solution
This is best explored by breaking down the way its components evaluate.
`[[]][0]` alone evaluates to the empty array `[]`. By adding `+` in front, you cast its string representation to an integer 0 (like saying `+4` or `-3`) via a unary positive operator. `+0` is just `0`.
`++` as a numeric operator, also casts the empty string to an integer `0`, but applies its operation (the prefix increment) resulting `1`.
[[]][0]
// [] empty array
[[]][0].toString()
// ""
// Unary + casts the empty string to an integer
+("")
// 0
// Prefix increment on an empty string results in 1 (increments the 0)
var emptyString = "";
++emptyString;
// 1
Problem
My question may be already answered, but I could not find it not in Search Engines google or bing doesn't like '+' (plus) sign in search request. Anyway, why this is zero ``` +[[]][0] // = 0 ``` and this is one ``` ++[[]][0] // = 1 ``` UPD: Michael Berkowski have a good answer, but I steal don't understand one thing if `[[]][0]` evaluates to an empty array, then why `++[]` is `ReferenceError: Invalid left-hand side expression in prefix operation` UPD2: now I get it.. it seems I was trying to type ++0 in console and getting an Error, but I should be using var a = 0; ++a