，--

++放前面先自增,后运算
++放后面先运算,再自增

var a = 3;
var b = ++a +4 + a++;
/*a=4  b = 8
 a=5    b=12
          */
console.log(a);  //5
console.log(b);  //12

<script>
  var i = 10;
   var b = 5 + i++;
   console.log('b=',b);  // 15
   console.log('i=',i);  // 11
</script>
<script>
    var i = 10;
    var b = 5 + ++i;
    console.log('b=',b);  // 16
    console.log('i=',i);  // 11
 </script>
<script>
    // 求b的值
    var i = 16;
    var j = 20; 
    //     20  16     18
    var b = j + i++ + ++i;  // 包含了5步运算
    console.log(b);  // 54
</script>
<script>
    var i = 1;
    var j = 1;
    var a = 10;
    var b = a + i++;
    // b = 11;  i = 2;
    //         11   3   3       2
    // var c = b + ++i + i++ + ++j;  
    // 求b和c的值 
    // b=11, c=19
</script>
<script>
    var sum = 10;
    sum += 5;
    console.log(sum);  // 15
</script>

-- 放前面先自减,后运算
-- 放后面先运算,再自减

var a = 12;
// a--;
 a=a-1;
console.log(a);

<script>
    var i = 1;
    var j = 1;
    // () => 自加和自减 => 算术运算 => 关系运算(比较运算) => 逻辑运算 => 赋值运算
    var sum = 20 + i++ + 'abc' || 123 - ++i + ++j + ++j + '123' == 123 * 2/4;   // '21abc' 
</script>