jrrio · April 23, 2018 15:06 · Apr 23, 2018
diff --git a/reduce_Object.md b/reduce_Object.md
@@ -0,0 +1,102 @@
+# Calculate the average of Object properties
+
+Here is our Object literal with some properties. What we need to do is calculate the average height of a set of people using JavaScript.
+
+```javascript
+const data = {
+  "Matt": { "height" : 176, "weight": 87 },
+  "Jason": { "height" : 190, "weight": 103 },
+  "Peter": { "height" : 180, "weight": 98 }
+};
+```
+
+## Using a for...in loop statement
+
+The value of the **data** constant is a reference to an Object instance, which inherits properties from *Object.prototype*. As the [for...in](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for...in) statement iterates over the enumerable properties of an Object, including those inherited from its prototype chain, we'll need to use the [hasOwnProperty()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/hasOwnProperty) method so that inherited properties are not considered.
+
+```javascript
+var count = 0, sumHeight = 0;
+for (var key in data) {
+ if (data.hasOwnProperty(key)) {
+   if (data[key].hasOwnProperty("height")) {
+     sumHeight += data[key].height;
+     count += 1;
+   }
+ }
+}
+console.log(sumHeight/count);  // 182
+```
+
+## Using Object.values() and the reduce() method
+
+The [Object.values()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/values) method returns an array of a given object's **own enumerable property** values, in the same order as that provided by a *for...in* loop (the difference being that a *for-in* loop enumerates properties in the prototype chain as well). For example, *Object.values(data)* will return the following array:
+
+```javascript
+[ { height: 176, weight: 87 },
+  { height: 190, weight: 103 },
+  { height: 180, weight: 98 } ]
+```
+
+The [reduce()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/Reduce) method applies a function against an accumulator and each element in the array (from left to right) to reduce it to a single value. This method will execute the callback function in different ways, depending on the existence of the optional *initialValue* argument.
+
+If we don't provide an *initialValue*, the callback function should return an Object for the next call within *reduce()*. For instance:
+
+```javascript
+var arr = Object.values(data);
+var sum = (prev, cur) => ({height: prev.height + cur.height});
+var avg = arr.reduce(sum).height / arr.length;
+console.log(avg); // 182
+```
+
+And if we provide an *initialValue* of 0, **reduce()** will invoke the callback function differently:
+
+```javascript
+var arr = Object.values(data);
+var sum = (accumulator, obj) => accumulator + obj.height;
+var avg = arr.reduce(sum, 0) / arr.length;
+console.log(avg); // 182
+```
+
+## Using Object.keys()
+
+The [Object.keys()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/keys) method returns an array that contains the names of the own enumerable properties of the object. For instance, *Object.keys(data)* will return the following array:
+
+```javascript
+['Matt', 'Jason', 'Peter']
+```
+
+So, let's write the code using that method:
+
+```javascript
+var arr = Object.keys(data);
+var getHeight = key => data[key]["height"];
+var avg = arr.reduce((a,c) => a + getHeight(c), 0) / arr.length;
+console.log(avg); // 182
+```
+
+## Using Object.entries()
+
+The [Object.entries()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/entries) method returns an array that contains the *[key, value]* pairs of the own enumerable properties of the object. Then using *Object.entries(data)* will return the following array:
+
+```javascript
+[ [ 'Matt', { height: 176, weight: 87 } ],
+  [ 'Jason', { height: 190, weight: 103 } ],
+  [ 'Peter', { height: 180, weight: 98 } ] ]
+```
+
+Now let's write the code using the array returned by the *Object.entries()* method:
+
+```javascript
+var heights = Object.entries(data).map(el => el[1].height);
+var avg2 = heights.reduce((a,c) => a + c) / heights.length;
+console.log(avg2); // 182
+```
+
+### References in MDN
+
+[1] [for...in](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for...in) statement  
+[2] [hasOwnProperty()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/hasOwnProperty) method  
+[3] [reduce()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/Reduce) method  
+[4] [Object.values()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/values) method  
+[5] [Object.keys()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/keys) method  
+[6] [Object.entries()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/entries) method
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