Add Type Narrowing With Similarly Shaped Objects as a typescript til

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

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+# 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).