JavaScript in parentheses () and square brackets [] special usage questions and Answers _ basics

(1, 2, 3);
return 3; Very curious, why return 3? What kind of mechanism is this? What is the effect of parentheses?

(1, 2, 3, alert) ("amazing!");
It's even more fun to call alert directly, which can be invoked by a global function.

The following are questions about square brackets []:
In JS, [] is almost equal to several paintings, but there are also magical times.

[1,2,3] [1];
return 2; Of course, this method is also used frequently, the index of the array and the response property name of the invoked object are stored in a variable.
[1,2,3] [1,2];

return 3; This is a lot more interesting, like the parentheses above, which returns the last value in parentheses.

In (1,2,3), parentheses and commas are operators, and the comma operator returns the result of the last expression, and the specific rules can be found by themselves.
("amazing") parentheses represent a function call.
Alert is a function object that, when returned as a result of an expression, can of course be invoked.

[1,2,3] Here the square brackets are declared array constants, immediately following [1] to represent the array element. [1,2,3] [1] means to take the first element in the array [1,2,3], that is, 2 (subscript starting from 0)

[1,2,3] [1,2]=[1,2,3][(1,2)]=[1,2,3][2]=3