Why does evaluating `[5,6,8,7][1,2]` equal 8 in JavaScript?

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

TypeScript and JavaScript may seem familiar to programmers who have previously worked with other programming languages such as C, Java, and Python. However, JavaScript features certain quirks that can be puzzling to even experienced programmers. One such quirk is the unexpected behavior of subscript operations with multiple indices, as demonstrated below:

<code class="javascript">[1,2,3,4,5,6][1,2,3]; // 4
[1,2,3,4,5,6][1,2]; // 3</code>

These examples show that when a subscript operation uses multiple indices, the result is determined by evaluating the expression formed by the indices. For instance, in the expression [1,2,3,4,5,6][1,2,3], the indices 1,2,3 are evaluated to form the expression 1 2 3 = 6, and the value at index 6 in the array is returned, which is 4.

To understand this behavior, it's crucial to recognize that the second set of square brackets is not part of an array definition. Instead, it represents a subscript operation, where the expression within the brackets determines the index of the element to be retrieved. Therefore, in the expression [5,6,8,7][1,2], the expression within the brackets evaluates to 1 2 = 3, indicating that the element at index 3 should be accessed. This element has the value 8.

This behavior can be confusing because in other programming languages like C, accessing an array element using an out-of-bounds index typically results in an exception or unpredictable behavior. In JavaScript, however, this behavior is allowed and the expression simply evaluates to undefined.

It's important to understand the underlying mechanics of this quirk to avoid unexpected results in your JavaScript code. By comprehending how subscript operations with multiple indices work, you can write more reliable and error-free code.

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