Why Does [5,6,8,7][1,2] = 8 in JavaScript?

Unveiling the Curious Case of JavaScript Array Subscripting

In the realm of JavaScript, certain quirks can leave developers scratching their heads. One such conundrum is the unexpected behavior of array subscript operations. Consider the following code:

[5,6,8,7][1,2] = 8

Intriguingly, this expression evaluates to 8, leaving many perplexed. To resolve this enigma, we delve into the internals of JavaScript array subscripting.

Array Subscripting with Multiple Arguments

The key to understanding this behavior lies in realizing that the second set of square brackets [] in [5,6,8,7][1,2] does not represent an array. Instead, it indicates an array subscript operation, where the expression inside the brackets becomes the index.

In this case, the expression 1,2 evaluates to 3. Substituting this value, we have:

[5,6,8,7][3] = 8

This expression now performs a regular array subscript operation, assigning the value 8 to the element at index 3.

The Comma Operator

The comma operator (`) plays a crucial role here. Its primary purpose is to separate multiple expressions, but it has an additional effect of evaluating each operand from left to right and returning the value of the last operand.

In the expression 1,2,3, the comma operator evaluates both 1 and 2, but the result of the entire expression is 3.

Conclusion

Armed with this knowledge, it becomes clear that the perplexing behavior is merely a result of array subscripting and the comma operator working together. By understanding the underlying mechanics, we can demystify this JavaScript quirk and avoid future confusion.

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