Special Use of parentheses () and square brackets [] in JavaScript _ basic knowledge

You may have some questions about the special usage of parentheses () and square brackets [] in JavaScript. This article will introduce these questions in detail, do not miss out on friends who have doubts (1, 2, 3 );
// Return 3; curious, why does it return 3? What mechanism is this? What is the role of parentheses?

(1, 2, 3, alert) ("Amazing! ");
// This is even more fun. You can call alert directly. After testing, global functions can be called in this way.

The following questions about square brackets:
In JS, [] is almost equal to the array, but there is also a magic time.

[1, 2, 3] [1];
// Return 2; of course, this method is also frequently used. It is often used when the index of an array and the response attribute name of the called object are stored in a variable.
[1, 2, 3] [1, 2];

// Return 3; this situation is much more interesting. It is similar to the situation in the parentheses above and returns the last value in the brackets.

In (, 3), brackets and commas are all operators. The comma operator returns the result of the evaluation of the last expression. You can check the specific rules by yourself.
("Amazing") brackets indicate function calls.
Alert is a function object. When it is returned as an expression evaluate result, it can certainly be called.

[, 3] square brackets are used to declare an array constant, followed by [1] to retrieve array elements. [, 3] [1] indicates the first element in the array [, 3], that is, 2 (subscript starts from 0)

[, 3] [] = [, 3] [()] = [, 3] [2] = 3