Javascript-For循环键仅在第一次迭代中未定义? [英] Javascript - For Loop key undefined ONLY for 1st iteration?

问题描述

为什么在array [j]的第一次迭代中总是打印UNDEFINED?

Why does this always print UNDEFINED for the first iteration of array[j]??

var array = [1,2,3,4,5]
for (i in array) {

    console.log("array[i]:" +array[i]);

    for (j=i+1; j<array.length-1;j++) {

        console.log("array[j]:"+ array[j]);
    }
}

推荐答案

这是因为i包含字符串值，而不是数字.

That's because i contains a string value, not a number.

将j设置为i+1会使它成为"0"+1，而不是0+1，因此是"01".由于array中没有键"01"的项目，因此您会返回undefined.

Setting j to i+1 makes it "0"+1, not 0+1, so that is "01". As there is no item in array with the key "01" you get undefined back.

执行j++时，它将把"01"转换为数字1并增加它，以便循环将按预期运行到最后.

When j++ is executed, it will convert the "01" into the number 1 and increase it, so that loop will run as expected to the end.

对于外循环中的下一个迭代，您得到j = "11"，并且将其与数组的长度进行比较时，它将跳过内循环.对于外循环中的其余迭代，由于j将在数组外部开始，因此将跳过内循环.

For the next iteration in the outer loop, you get j = "11", and when that is compared to the length of the array it will skip the inner loop. For the rest of the iterations in the outer loop, the inner loop will be skipped as j will start outside of the array.

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