Add for...in Iterates Over Object Properties as a javascript til

README.md

@@ -10,7 +10,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 For a steady stream of TILs from a variety of rocketeers, checkout
 [til.hashrocket.com](https://til.hashrocket.com/).
 
-_751 TILs and counting..._
+_752 TILs and counting..._
 
 ---
 
@@ -265,6 +265,7 @@ _751 TILs and counting..._
 - [Easy Date Comparison With DayJS](javascript/easy-date-comparison-with-dayjs.md)
 - [Expand Emojis With The Spread Operator](javascript/expand-emojis-with-the-spread-operator.md)
 - [Fill An Input With A Ton Of Text](javascript/fill-an-input-with-a-ton-of-text.md)
+- [for...in Iterates Over Object Properties](javascript/for-in-iterates-over-object-properties.md)
 - [Freeze An Object, Sorta](javascript/freeze-an-object-sorta.md)
 - [Get The Location And Size Of An Element](javascript/get-the-location-and-size-of-an-element.md)
 - [Get The Time Zone Of The Client Computer](javascript/get-the-time-zone-of-the-client-computer.md)

javascript/for-in-iterates-over-object-properties.md

@@ -0,0 +1,30 @@
+# for...in Iterates Over Object Properties
+
+I don't reach for _for loops_ very often, so when I needed one recently I
+thought I'd check out the newer `for...in` construct. It didn't behave quite
+how I was expecting. I thought it would iterate over the values in the
+target list, instead it seemed to be iterating over the indices.
+
+The [MDN
+docs](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for...in)
+explain what is going on:
+
+> The `for...in` statement iterates over all non-Symbol, enumerable properties
+> of an object.
+
+An array is an object whose properties are the indices of the values stored
+in the array.
+
+```javascript
+const fruits = ["apple", "banana", "orange"];
+for (let fruit in fruits) {
+  console.log(fruit, fruits[fruit]);
+}
+// => "0" "apple"
+// => "1" "banana"
+// => "2" "orange"
+```
+
+The iteration value wasn't what I was looking for, but in this case I can
+use it to access the value from the list. I'd be better off using a standard
+_for loop_ though.