a += b; == a + b = a;
a와 b를 더한 후 a에 결과값을 재할당
<script>
let a = 4;
a += 3; // a(4) + 3 = a(7)
a += 3; //a(7) + 3 = a(10)
a += 3; //a(10) + 3 = a(13)
a += 10; //a(13) + 3 = a(23)
</script>
<script>
let b = 5;
b -= 2;//b(5) - 2 = b(3)
b -= 5;//b(3) - 5 = b(-2)
</script>
<script>
let c = 25;
c -=3;//c(25) - 3 = c(22)
c *= 2;//c(22) * 2 = c(44)
c /= 4;//c(44) / 4 = c(11)
c %= 3;//c(11) % 3 = c(2)
c **= 3;//c(2) ** 3 = c(8)
</script>
<script>
let d = 10;
d += 5;//d(10) + 5 = d(15)
d /= 5;//d(15) / 5 = d(3)
d **= 5;//d(3) ** 5 = d(243)
d %= 5;//d(243) % 5 = d(3)
d *= 2;//d(3) * 2 = d(6)
</script>