Merge pull request #314 from Microsoft/nullUndefined

Null and Undefined

Author: DanielRosenwasser

pages/Basic Types.md

@@ -190,6 +190,27 @@ Declaring variables of type `void` is not useful because you can only assign `un
 let unusable: void = undefined;
 ```
 
+# Null and Undefined
+
+In TypeScript, both `undefined` and `null` actually have their own types named `undefined` and `null` respectively.
+Much like `void`, they're not extremely useful on their own:
+
+```ts
+// Not much else we can assign to these variables!
+let u: undefined = undefined;
+let n: null = null;
+```
+
+By default `null` and `undefined` are subtypes of all other types.
+That means you can assign `null` and `undefined` to something like `number`.
+
+However, when using the `--strictNullChecks` flag, `null` and `undefined` are only assignable to `void` and their respective types.
+This helps avoid *many* common errors.
+In cases where you want to pass in either a `string` or `null` or `undefined`, you can use the union type `string | null | undefined`.
+Once again, more on union types later on.
+
+> As a note: we encourage the use of `--strictNullChecks` when possible, but for the purposes of this handbook, we will assume it is turned off.
+
 # Type assertions
 
 Sometimes you'll end up in a situation where you'll know more about a value than TypeScript does.

pages/Compiler Options in MSBuild.md

@@ -80,7 +80,7 @@ Compiler Option                              | MSBuild Property Name
 `--suppressImplicitAnyIndexErrors`           | TypeScriptSuppressImplicitAnyIndexErrors   | boolean
 `--target`                                   | TypeScriptTarget                           | `ES3`, `ES5`, or `ES6`
 `--traceResolution`                          | *Not supported in MSBuild*                 |
-`--types`                                    | *Not supported in MSBuild*                 | 
+`--types`                                    | *Not supported in MSBuild*                 |
 `--typeRoots`                                | *Not supported in MSBuild*                 |
 `--watch`                                    | *Not supported in MSBuild*                 |
 *MSBuild only option*                        | TypeScriptAdditionalFlags                  | *Any compiler option*