JS array traversal method for loop and for...in_javascript techniques

There are two ways to traverse JS arrays:

The first type: general for loop, for example:

var a = new Array("first", "second", "third") 
for(var i = 0;i < a.length; i++) {
document.write(a[i]+",");
}

Output results: fitst, second, third

The first method: use for...in this traversal method, for example:

var arr = new Array("first", "second", "third") 
for(var item in arr) {
document.write(arr[item]+",");
}

Output results: fitst, second, third