Added TypeScript/Unknown Type (#26181)

guide/english/typescript/unknown-type/index.md

@@ -0,0 +1,31 @@
+---
+title: Unknown Type
+---
+## Undefined Type
+
+In TypeScript, `unknown` type is a type-safe counterpart of `any`, where anything can be assigned to `unknown` but cannot assign to anything except itself and `any`.
+
+```typescript
+const unknownValue = 1
+
+let x: unknown = unknownValue
+x = 2 // type-safe
+
+const y: number = x // type-safe fails
+```
+
+An `unknown` type should be casted to a specific type to be used.
+
+```typescript
+const matchUnknown = (x: unknown) => {
+    if (typeof x === 'string') {
+        return x.toUpperCase()
+    }
+
+    if (typeof x === 'number') {
+        return x * 2
+    }
+
+    throw new Error(`type of ${x} is not supported`)
+}
+```