Add enum example using keyof typeof

pages/Enums.md

@@ -223,6 +223,32 @@ function f(obj: { X: number }) {
 f(E);
 ```
 
+## Enums at compile time
+
+Even thought Enums are real objects that exist at runtime, the `keyof` keyword works differently than you might expect for typical objects. Instead, use `keyof typeof` to get a Type that represents all Enum keys as strings.
+
+```ts
+enum LogLevel {
+    ERROR, WARN, INFO, DEBUG
+}
+
+/**
+ * This is equivalent to:
+ * type LogLevelStrings = 'ERROR' | 'WARN' | 'INFO' | 'DEBUG';
+ */
+type LogLevelStrings = keyof typeof LogLevel;
+
+function printImportant(key: LogLevelStrings, message: string) {
+    const num = LogLevel[key];
+    if (num <= LogLevel.WARN) {
+       console.log('Log level key is: ', key);
+       console.log('Log level value is: ', num);
+       console.log('Log level message is: ', message);
+    }
+}
+printImportant('ERROR', 'This is a message');
+```
+
 ### Reverse mappings
 
 In addition to creating an object with property names for members, numeric enums members also get a *reverse mapping* from enum values to enum names.