var arrayOfNumbers=[1,2,3,4,5,6,78];
for(var index in arrayOfNumbers){
   console.log(index+1);
}

Note: for…in should not be used to iterate over an Array where the index order is important.

… The for…in loop statement will return all enumerable properties,
including those with non–integer names and those that are inherited.

当在键中使用for …时总是一个字符串,它所做的只是字符串连接.

你有一个数组,所以最好像这样使用Array.foreach()：

var arrayOfNumbers=[1,78];
arrayOfNumbers.forEach(function(item,index){
     console.log(index + 1); // here the index is a number!
});