数组中的javascript for-in循环

Question

If the code is this:

arr=Array("a","b","c");
for(i in arr);
{
 alert(i);
}

there is no any alert,but if it is this:

arr=new Array("a","b","c");
for(i in arr)
{
 alert(i);//alerts 0,1,2
}

What is the reason?

Answer 1

Array is a constructor. To create a new object you should be using the new operator to create the object, which the constructor is then bound to and run against. In this case though, it actually should work either way, your issue is most likely related to the semicolons next to your for-loop, as noted in the comments.

As an aside, for creating a new array its generally advised to use the simpler notation

var arr = ["a","b","c"];

Its also questionable to use a for-in loop with an array in javascript, as that will hit any additional properties defined on the array. (so if you said arr.x = 2 it would also iterate over x .

Better to use the iterative form

var i =0, length =arr.length;
for ( ;i<length; i++) {

    alert(arr[i]);
}

Answer 2

The reason you're getting different results is that you were using incorrect syntax for your for/in loops.

for(i in arr);
{
 alert(i);
}

should not have the first semicolon.

Also note that a nicer way to iterate over an array would be:

arr.forEach(function(value, index){
    alert(value); // or alert(index);
});

As bfavaretto has mentioned.

Invoking the Array function without the new keyword will Create and return a new Array object in the same way it would had you used the new keyword.

So these two will alert the same things:

arr1 = new Array("a","b","c");
arr2 = Array("a","b","c");