Create ts-exact-optional-property-types.md

learnings/ts-exact-optional-property-types.md

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+# TypeScript: exactOptionalPropertyTypes
+
+TypeScript handles optional and undefined differently - [exactOptionalPropertyTypes](https://www.typescriptlang.org/tsconfig#exactOptionalPropertyTypes) will help catch scenarios where `undefined` should be typed explicitly.
+
+When turned off:
+
+```ts
+type User = {
+  email?: string;
+}
+
+const user: User = { email: undefined }
+// Valid
+```
+
+When turned on:
+
+```ts
+type User = {
+  email?: string;
+}
+
+const user: User = { email: undefined }
+    // Type '{ email: undefined; }' is not assignable to type 'User' with 'exactOptionalPropertyTypes: true'.
+    // Consider adding 'undefined' to the types of the target's properties.
+```
+
+Read [optional vs undefined | TkDodo's blog](https://tkdodo.eu/blog/optional-vs-undefined) to learn the difference and how this can help.