Add Type Narrowing With Similarly Shaped Objects as a typescript til

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).
 
-_1108 TILs and counting..._
+_1109 TILs and counting..._
 
 ---
 
@@ -1008,6 +1008,7 @@ _1108 TILs and counting..._
 - [Add Types To An Object Destructuring](typescript/add-types-to-an-object-destructuring.md)
 - [Compiler Checks For Unused Params And Variables](typescript/compiler-checks-for-unused-params-and-variables.md)
 - [Re-Export An Imported Type](typescript/re-export-an-imported-type.md)
+- [Type Narrowing With Similarly Shaped Objects](typescript/type-narrowing-with-similarly-shaped-objects.md)
 - [Zero-Config Environments For Trying Out Types](typescript/zero-config-environments-for-trying-out-types.md)
 
 ### Unix

typescript/type-narrowing-with-similarly-shaped-objects.md

@@ -0,0 +1,46 @@
+# Type Narrowing With Similarly Shaped Objects
+
+Let's say we have a type with several properties and a variable of that type.
+
+```typescript
+type User = {
+  firstName: string
+  lastName: string
+  age: number
+  email: string
+}
+
+const liz: User = {
+  firstName: 'Liz',
+  lastName: 'Lemon',
+  age: 38,
+  email: 'liz@example.com'
+}
+```
+
+We can use variables of this type in _narrower_ contexts as long as the
+properties that are present have aligning types.
+
+For instance, we can pass a `User` to this `sendNewsletter` function. Even
+though the types don't match exactly, the type of `sendNewsletter`'s parameter
+is a subset of `User`.
+
+```typescript
+const sendNewsletter = ({
+  firstName,
+  email,
+}: {
+  firstName: string;
+  email: string;
+}) => {
+  console.log(`Sending newsletter to ${firstName} at ${email}`);
+};
+
+sendNewsletter(liz);
+// "Sending newsletter to Liz at liz@example.com"
+```
+
+This is a form of [_type
+narrowing_](https://www.typescriptlang.org/docs/handbook/2/narrowing.html)
+through [structural
+subtyping](https://www.typescriptlang.org/docs/handbook/type-compatibility.html).