var i,j=5,k;
for(i=0; i<10; i++,j++) {
    k = i + j;
}
console.log(i);
console.log(j);
console.log(k);
//9,15,23
var a = 20;
var b = (++a, 10);
console.log(a);
console.log(b);
//21,10
console.log((2,5));
//5
//加括號提升逗號運算符優先級

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