Add Use An Array Check For Type Narrowing as a typescript til

Author: jbranchaud

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).
 
-_1110 TILs and counting..._
+_1111 TILs and counting..._
 
 ---
 
@@ -1013,6 +1013,7 @@ _1110 TILs and counting..._
 - [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)
+- [Use An Array Check For Type Narrowing](typescript/use-an-array-check-for-type-narrowing.md)
 - [Zero-Config Environments For Trying Out Types](typescript/zero-config-environments-for-trying-out-types.md)
 
 ### Unix

typescript/use-an-array-check-for-type-narrowing.md

@@ -0,0 +1,33 @@
+# Use An Array Check For Type Narrowing
+
+If you are typing a concatenation function for melding two values together into
+a single array, you may end up with a function signature like this:
+
+```typescript
+type ConcatFunction = (value: any | any[], array: any[]) => any[];
+```
+
+That first argument can be an individual value or an array of values. You'll
+need to handle both scenarios in the function implementation. Using the
+[`Array.isArray`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/isArray)
+function as a _type guard_, you can check differentiate between those two
+cases.
+
+```typescript
+const concat: ConcatFunction = (value, array) => {
+    if(Array.isArray(value)) {
+        return [...value, ...array];
+    } else {
+        return [value, ...array];
+    }
+}
+
+concat(true, [1,2,3]);
+// [true, 1, 2, 3]
+
+concat([1,2,3], ['a', 'b', 'c'])
+// [1, 2, 3, 'a', 'b', 'c']
+```
+
+This is a form of [type
+narrowing](https://www.typescriptlang.org/docs/handbook/2/narrowing.html).