Why Does JavaScript Produce Unexpected Results in String and Number Concatenation and Arithmetic?

JavaScript Quirks Unraveled: Explaining the Oddities from the 'Wat' Talk

Empty Array Empty Array: The Emptiness Concatenation

When adding two empty arrays, JavaScript converts them to primitives first. In this case, the default value of an array is an empty string, so the result is the concatenation of two empty strings: an empty string.

Empty Array Object: The Object Sanitization

Converting the empty array to a primitive (empty string) and the object to a primitive (its string representation, "[object Object]") results in the concatenation of an empty string and an object representation, yielding "[object Object]."

Object Empty Array: JavaScript's Unconventional Arithmetic

Unlike the video, in the provided JSFiddle, the result of {} [] is [Object] instead of 0. This discrepancy arises because the empty block {} is interpreted as a block statement, which evaluates to empty. The unary operator converts the empty value to a number, resulting in 0. However, since you're using this within a function argument, the statement is forced to be interpreted as an expression statement. This causes {} to be parsed as an empty object literal instead of an empty block, leading to [Object].

Object Object: A Mismatched Output

The result of {} {} should be "object Object," but the JSFiddle shows a different result. This discrepancy is likely because of JavaScript's implementation-specific behavior where the result of adding two objects to a string can vary.

Array(16).join("wat" - 1): NaN Propagation

The input string for this expression is "NaN-1," which evaluates to NaN. When JavaScript tries to concatenate "NaN" with the array's empty string elements, the result is always NaN according to the concatenation algorithm.

"wat" 1 vs. "wat" - 1: The String vs. Number Conversion

In "wat" 1, JavaScript converts 1 to a string, resulting in "wat1." On the other hand, in "wat" - 1, it attempts to convert "wat" to a number, which results in NaN. Since the concatenation operator requires a string as its first argument, NaN is converted back to a string, resulting in "NaN" being repeated.

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