for...in vs. for

Summary: The for...in-loop counts only the used indexes in an array while the "regular" for-loop loops through the entire thing.

Setup:

anArray = [];
anArray[50] = "Fifty";
anArray[100] = "Hundred";
anArray[150] = "Hundred Fifty";


console.log(anArray.length); // 151

Way to count actual used elements efficiently.

var count  = 0;
for (var index in anArray) {

    console.log(index);
    /*
        Loop 1: 50
        Loop 2: 100
        Loop 3: 150
    */

    count++;

    console.log(anArray[index]);
    /*
        Loop 1: "Fifty"
        Loop 2: "Hundred"
        Loop 3: "Hundred Fifty"
    */
}
console.log(count); // 3