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

Author: jbranchaud

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
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-_1184 TILs and counting..._
+_1185 TILs and counting..._
 
 ---
 
@@ -1081,6 +1081,7 @@ _1184 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)
+- [Interfaces With The Same Name Are Merged](typescript/interfaces-with-the-same-name-are-merged.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)
 - [Use An Array Check For Type Narrowing](typescript/use-an-array-check-for-type-narrowing.md)

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

@@ -0,0 +1,39 @@
+# 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)