Merge branch 'undefined'

Author: Kikobeats

docs/_sidebar.md

@@ -42,3 +42,4 @@
 	* [Scope](workflow/scope.md)
 	* [Variable access](workflow/variable-access.md)
 	* [How to clone](workflow/how-to-clone.md)
+	* [null and undefined](workflow/null-and-undefined.md)

docs/workflow/null-and-undefined.md

@@ -0,0 +1,15 @@
+# null and undefined
+
+While `null` means "no object", `undefined` is "no value".
+
+The simple way for typecheck both value is using weak equal:
+
+```js
+const isNil = value => value == null
+```
+
+Although the problem is still there: everyone uses `null` and `undefined` inconsistently and interchangeably, not being clear when to use which.
+
+We cannot remove `null` from JavaScript (e.g., JSON doesn't support `undefined`) but we can mitigate the impact.
+
+In order to apply a simple rule there, just [use `undefined` over `null`](https://github.com/Microsoft/TypeScript/wiki/Coding-guidelines#null-and-undefined).