javascript中的`(,)`和`,`是什么?

Operator Precedence

var a = 42, b;
b = ( a++, a );

a;  // 43
b;  // 43

( )

var a = 42, b;
b = a++, a;

a;  // 43
b;  // 42

, = b = a++, a (b = a++), a a++ b ++ a

() Operator precedence on MDN

() b = ( a++, a );

,

var a = 42, b;

var a = 42, 
var b;

mozilla

var x = 1;

x = (x++, x);

console.log(x);
// expected output: 2

x = (2, 3);

console.log(x);
// expected output: 3