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

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

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+# 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).