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

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

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+# 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.