Hash maps with no side effects

_posts/en/javascript/2017-xx-xx-hash-maps-without-side-effects

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+---
+layout: *post
+
+title: Hash maps without side effects
+tip-number: xx
+tip-username: bhaskarmelkani
+tip-username-profile: https://www.twitter.com/bhaskarmelkani
+tip-tldr: Create hash maps(without side effects) using `Object.create(null)`.
+
+categories:
+    - en
+    - javascript
+---
+When you want to use javascript object as a hash map(purely for storing data), you might want to create it as follows.
+
+```js
+  const map = Object.create(null);
+```
+
+When creating a map using object literal(`const map = {}`), the map inherites properties from Object by default. It is equivalent to `Object.create(Object.prototype)`.
+
+But by doing `Object.create(null)`, we explicitly specify `null` as its prototype. So it have absolutely no properties, not even constructor, toString, hasOwnProperty, etc. so you're free to use those keys in your data structure if you need to.
+
+**Rationale:-**
+```js
+const dirtyMap = {};
+const cleanMap = Object.create(null);
+
+dirtyMap.constructor    // function Object() { [native code] }
+
+cleanMap.constructor    // undefined
+
+//iterating maps
+
+const key;
+for(key in dirtyMap){
+  if (dirtyMap.hasOwnProperty(key)) {   // Check to avoid iterating over inherited properties.
+    console.log(key + " -> " + dirtyMap[key]);
+  }
+}
+
+for(key in cleanMap){
+  console.log(key + " -> " + cleanMap[key]);    // No need to add extra checks, as the object will always be clean
+}
+```
+
+**Note**: 
+* Object.create() was introduced in ES5 [[Compatibility]](http://kangax.github.io/compat-table/es5/)
+* ES6 introduced some new structures: [Map](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Map), [WeakMap](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WeakMap), [Set](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set) and [Weak Set](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WeakSet)