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

Exploring the Enigma of " [[]][ []] [ []]" in JavaScript

JavaScript developers may find themselves stumped by the expression " [[]][ []] [ []]" as it yields the unexpected result of "10". Let's delve into the process behind this enigmatic calculation to unravel its intricacies.

Breaking It Down

If we break down the expression, it reads as follows:

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

In JavaScript, array elements can be coerced into numbers using the ' ' operator. Specifically, [] === 0. This coerces arrays into empty strings, resulting in "" === 0.

Simplifying the Computation

With this understanding, we can simplify the expression as:

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

Accessing the first element of an array, [[]][0], returns the inner array. Due to references, we'll refer to it as A to avoid confusion.

Incrementing and Converting

The operator increments its operand by one and returns the incremented result. Therefore, [[]][0] is equivalent to Number(A) 1.

String Concatenation

We can further simplify:

(+[] + 1)
+
[0]

First, [] is coerced into the string "0". Then, [] results in 1. Finally, "1" is concatenated with "0" using , as arrays can be joined with , resulting in 10.

Technicalities

Behind the scenes, the process of coercing [] into 0 involves several specification references:

• Unary Operator: Converts its operand to a number, coercing it into a string ("") first.
• Number Conversion (ToNumber()): Coerces the array ([]) into a string ("").
• Primitive Value Conversion (ToPrimitive()): In String mode, uses the object's [[DefaultValue]] method.
• [[DefaultValue]]: Calls the [[Get]] method with argument "toString".
• Array's toString Method: Invokes the [[Join]] method, which yields "" for [].

Conclusion

Through this step-by-step analysis, we have shed light on the mechanics behind the baffling expression " [[]][ []] [ []]" in JavaScript. Its execution involves coercion, incrementation, and concatenation, ultimately revealing the logical journey that leads to the surprising result of "10".

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