Add enum example using keyof typeof

[styfle]

I always forget this syntax.

Please add it to the handbook.

Fixes microsoft/TypeScript#14106

[tjallingt]

Should probably include a note that this doesn't work for enums with custom values:

enum LogLevel {
  ERROR = 'error',
  WARN = 'warn',
  INFO = 'info',
  DEBUG = 'debug',
}

since the type LogLevelStrings will be 'ERROR' | 'WARN' | 'INFO' | 'DEBUG' instead of 'error' | 'warn' | 'info' | 'debug'

[styfle]

@tjallingt It works for string enums too. I reworded the description to be clear that this is getting the keys and not the values.

See this example and try running the JS output.

[tjallingt]

Great! I find the updated text much clearer 👍
And yea i meant to say that the way it works is a little bit less clear when concerning enums with custom values. (Yea it says keyof but it doesn't hurt being explicit about what part is actually the key ;p)

[orta]

Cool, thanks!