Why does [[]][ []] [ []] return '10' in JavaScript?

Unveiling the Mystery Behind [[]][ []] [ []]'s Return Value

In the realm of JavaScript, the enigmatic expression [[]][ []] [ []] perplexes many with its seemingly nonsensical return value: "10". Delving into the depths of JavaScript's evaluation process, we embark on a journey to decipher this puzzle.

Initially, let's break down the expression:

++[[]][+[]]
+
[+[]]

Expanding [[]][ []]

In JavaScript, the expression [] evaluates to 0. This is because attempts to convert the operand into a number, and an empty array coerces to a string, which is then converted to a number resulting in 0.

Substituting [] with its equivalent, we get:

++[[]][0]
+
[+[]]

Evaluating [[]][0]

The operator increments its operand by one, so [[]][0] is equivalent to ([[]][0]) 1. Since [[]][0] retrieves the first element of the empty array, it returns another empty array. Thus, ([[]][0]) is equivalent to [], which we know yields 0.

Simplifying the Remaining Expression

Simplifying further, we obtain:

1
+
[+[]]

JavaScript's Coercion Magic

JavaScript's coercion rules come into play when dealing with the addition of an array and a number. First, the array coerces to a string ("0"), and then the number coerces to a string ("1"). String concatenation results in the final outcome:

console.log("1" + "0") // "10"

Additional Context

• The expression [[]] can be understood as a short way to coerce an array to a number (0).
• The [] operation is not explicitly defined in the JavaScript specification, but it's the result of a series of coercion steps.
• JavaScript's coercion rules can lead to unexpected behavior in certain scenarios, but they offer flexibility in converting values to different types.

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