Add Interfaces With The Same Name Are Merged as a TypeScript til

typescript/interfaces-with-the-same-name-are-merged.md

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+# Interfaces With The Same Name Are Merged
+
+Here is the declartion of an interface in TypeScript.
+
+```typescript
+interface Person {
+  name: string
+}
+```
+
+What if I were to add a separate interface declaration with the same name,
+`Person`?
+
+```typescript
+interface Person {
+    age: number
+}
+```
+
+TypeScript performs declaration merging. So the types of the two interfaces
+would be combined. So, a variable of type `Person` can have an `name` and an
+`age`.
+
+```typescript
+const person: Person = {
+    age: 22,
+    name: 'Bob'
+}
+```
+
+See a [live
+example](https://www.typescriptlang.org/play?ssl=12&ssc=2&pln=5&pc=1#code/JYOwLgpgTgZghgYwgAgArQM4HsTIN4BQyxyIcAthAFzIZhSgDmBAvgQaJLIiulNrkIlkcRtVIBXcgCNordghx1kAB0w4afAcgC8+IiVHiATMYA0B4mUo0A5ACEs02-IJr+OAHRGgA)
+in the TS Playground.
+
+This is different from how object type declarations handle it. If I were to try
+to define two separate `type`s with the same name, that would result in a type
+error.
+
+[source](https://www.typescriptlang.org/docs/handbook/declaration-merging.html#merging-interfaces)