Add Add Generic Typing To An Anonymous Function as a TypeScript TIL

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
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-_1291 TILs and counting..._
+_1292 TILs and counting..._
 
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@@ -1191,6 +1191,7 @@ _1291 TILs and counting..._
 
 ### TypeScript
 
+- [Add Generic Typing To An Anonymous Function](typescript/add-generic-typing-to-an-anonymous-function.md)
 - [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)
 - [Create A Non-Empty Array Type](typescript/create-a-non-empty-array-type.md)

typescript/add-generic-typing-to-an-anonymous-function.md

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+# Add Generic Typing To An Anonymous Function
+
+A common pattern, especially when dealing with collections of data (read:
+arrays), is to have a function that can transform an array of any type of data.
+In order to keep this kind of function general-purpose and have it preserve
+types, we'll need to use generics.
+
+For an anonymous function to work with a generic type, it needs to open with a
+signature for the generic type:
+
+```typescript
+<T>() => {}
+```
+
+With that in mind, we can write a function that accepts an array of type `T`,
+does something to that collection, and then returns an array of type `T`.
+
+For instance, here is a function that takes the first `n` elements from the
+array and returns them.
+
+```typescript
+const take = <T>(arr: Array<T>, n: number): Array<T> => {
+  return arr.slice(0, n)
+}
+```
+
+This both enforces and preserves the type of the array. It works well for
+situations where the array is a bunch of complex objects with specific types
+like we'd get from a Prisma query.