for/in Loops

Another type of for loop is the for/in loop. The for/in loop executes on any data type that can be iterated. For the most part, you will use for/in loops on arrays and objects. The following example illustrates the syntax and behavior of the for/in loop on a simple array:

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var days = ["Monday", "Tuesday", "Wednesday", "Thursday", "Friday"];
for (var idx in days){
  console.log("It's " + days[idx] + "<br>");
}

Notice that the variable idx is adjusted each iteration through the loop, from the beginning array index to the last. The resulting output is:

It's Monday
It's Tuesday
It's Wednesday
It's Thursday
It's Friday