一道js的前端面试题，主要弄清楚逗号表达式的作用

群里看到的一道前端JS面试题。我以为我会，其实我错了。找了很多资料，写下来。

 var i,j,k;
 for( i = 0 , j = 0; i < 10 , j < 6; i++ , j++ ) {
 k = i+j;
 }
 document.write(k); //

控制台结果：

aaarticlea/png;base64,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" alt="" />

然后我把j修改成16.

     for(var i = 0, j = 0; i < 10 , j < 16 ; i++ , j++){
         k = i + j;
         console.log('i='+i+'，j='+j+'k的结果是：'+k);
     }
     console.log(k);//30

控制台结果：

aaarticlea/png;base64,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" alt="" />

我看到这个结果，感到不理解。然后查资料，有人说是因为：“这个主要是逗号语句，当有多个条件，之间用逗号隔开的时候，会返回最右侧的条件”

然后我又该改改代码：

     for (var i = 0,j = 0; i < 16; i++, j++)
     {
         k = i + j;
         console.log('i='+i+'，j='+j+'  k的结果是：'+k);
     }
     console.log(k);

控制台截图：

aaarticlea/png;base64,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" alt="" />

发现和第二种效果是等价的。

　　　　  var t = (4,5,7); console.log(t);//

一、逗号运算符的特性及作用 
逗号运算符的作用是将若干表达式连接起来。它的优先级别在所有运算符中是最低的，结合方向是"自左至右"的。 
如：3*3，4*4 
二、逗号表达式 
逗号表达式的一般形式是：表达式1，表达式2，表达式3……表达式n 
逗号表达式的求解过程是：先计算表达式1的值，再计算表达式2的值，……一直计算到表达式n的值。最后整个逗号表达式的值是表达式n的值。 
看下面几个例子： 
x=8*2,x*4 /*整个表达式的值为64，x的值为16*/ 
(x=8*2,x*4),x*2 /*整个表达式的值为128，x的值为16*/ 
x=(z=5,5*2) /*整个表达式为赋值表达式，它的值为10，z的值为5*/ 
x=z=5,5*2 /*整个表达式为逗号表达式，它的值为10，x和z的值都为5*/ 
逗号表达式用的地方不太多，一般情况是在给循环变量赋初值时才用得到。所以程序中并不是所有的逗号都要看成逗号运算符，尤其是在函数调用时，各个参数是用逗号隔开的，这时逗号就不是逗号运算符。 
如：printf("
%d,%d,%d",x,y,z);

###########################################################

运算符使它两边的表达式以从左到右的顺序被执行，并获得右边表达式的值。, 运算符最普通的用途是在 for 循环的递增表达式中使用。例如：
for (i = 0; i < 10; i++, j++)
{
k = i + j;
}

console.log(k);//18
每次通过循环的末端时， for 语句只允许单个表达式被执行。, 运算符被用来允许多个表达式被当作单个表达式，从而规避该限制。