Add 'prefer null' to frontend style guide
Change-Id: I8ecf85d0dbcf8893e5d5e75109870f1d1dd9102f
diff --git a/polygerrit-ui/FE_Style_Guide.md b/polygerrit-ui/FE_Style_Guide.md
index c9a5d9b..a636119 100644
--- a/polygerrit-ui/FE_Style_Guide.md
+++ b/polygerrit-ui/FE_Style_Guide.md
@@ -10,11 +10,20 @@
Additionally to the rules above, Gerrit frontend uses the following rules (some of them have automated checks,
some don't):
+- [Prefer null over undefined](#prefer-null)
- [Use destructuring imports only](#destructuring-imports-only)
- [Use classes and services for storing and manipulating global state](#services-for-global-state)
- [Pass required services in the constructor for plain classes](#pass-dependencies-in-constructor)
- [Assign required services in a HTML/Polymer element constructor](#assign-dependencies-in-html-element-constructor)
+## <a name="prefer-undefined"></a>Prefer `undefined` over `null`
+
+It is more confusing than helpful to work with both `null` and `undefined`. We prefer to only use `undefined` in
+our code base. Try to avoid `null`.
+
+Some browser and library APIs are using `null`, so we cannot remove `null` completely from our code base. But even
+then try to convert return values and leak as few `nulls` as possible.
+
## <a name="destructuring-imports-only"></a>Use destructuring imports only
Always use destructuring import statement and specify all required names explicitly (e.g. `import {a,b,c} from '...'`)
where possible.
