Add Always Use Inline Functions With Assign as an xstate til

Author: jbranchaud

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://tinyletter.com/jbranchaud).
 
-_1144 TILs and counting..._
+_1145 TILs and counting..._
 
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@@ -1336,6 +1336,7 @@ _1144 TILs and counting..._
 
 ### XState
 
+- [Always Use Inline Functions With Assign](xstate/always-use-inline-functions-with-assign.md)
 - [Define Event That Does Internal Self Transition](xstate/define-event-that-does-internal-self-transition.md)
 - [Events Stop Propagating Once Handled](xstate/events-stop-propagating-once-handled.md)
 - [Inline Actions vs Actions In Machine Options](xstate/inline-actions-vs-actions-in-machine-options.md)

xstate/always-use-inline-functions-with-assign.md

@@ -0,0 +1,50 @@
+# Always Use Inline Functions With Assign
+
+The [`assign`
+action](https://xstate.js.org/docs/guides/context.html#assign-action) allows
+you to update the context of the state machine.  There are a couple ways to use
+`assign`, but you should prefer the inline function style.
+
+Technically `assign` can be passed an object literal like so to set a static
+value:
+
+```javascript
+{
+  actions: {
+    resetCount: assign({
+      count: 0
+    })
+  }
+}
+```
+
+However, this causes confusion for the TypeScript compiler and can create
+surprising and unrelated type errors in the parts the code that use the
+machine.
+
+To keep the compiler happy, you should instead use inline functions. Either
+like this:
+
+```javascript
+{
+  actions: {
+    resetCount: assign((_context) => {
+      return {
+        count: 0
+      }
+    })
+  }
+}
+```
+
+or like this:
+
+```javascript
+{
+  actions: {
+    resetCount: assign({
+      count: (_context) => 0
+    })
+  }
+}
+```