Why Does [[]][ []] [ []] Result in the String '10' in JavaScript?

Unexpected String Concatenation in JavaScript

In JavaScript, the peculiar expression [[]][ []] [ []] surprisingly yields the string "10". Understanding the intricate process behind this behavior requires breaking down its individual components:

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

Unveiling the First Component: [[]]

The prefix increment operator increments its operand by one and returns the incremented result. In this case, the operand is [[]], which evaluates to the empty array ([]). Incrementing an array is not logical, but JavaScript accommodates such cases by converting the array into a number using the operator.

Understanding the Conversion: [[]]

The operator, when applied to an array, attempts to convert it into a number. However, an empty array evaluates to a falsehood, which gets coerced into the number 0. Thus, [[]] becomes equivalent to 0, or simply 0.

The Second Component: [ []]

Following the same logic, [ []] also converts the empty array into 0.

Bringing it Together: (0 1) [0]

The incremented expression becomes 1 (0 1), which is then added to [0]. In JavaScript, arrays can be coerced into strings by joining their elements with commas. Therefore, [0] is equivalent to "0" (joining an array with one element results in the element itself).

Coercing Numbers to Strings:

The expression now becomes 1 "0", which JavaScript attempts to concatenate as strings. The result is "10".

In-depth Analysis of Type Coercions:

• [] coerces the empty array into 0 (Number).
• [] coerces the empty array into 0 (Number).
• Incrementing by one results in 1 (Number).
• Joining the array [0] produces "0" (String).
• Adding the numbers 1 and 0 coerces them into strings, resulting in "1" and "0," respectively.
• Concatenating the strings yields "10".

Despite its seemingly complex appearance, the expression adheres to the precedence rules of JavaScript operators, with having higher precedence than . Understanding these precedence rules is crucial for debugging such expressions effectively.

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