Javascript编译器行为 - 对于空数组和零数组的数组加倍是.. ONE

Question

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

Answer 1

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