String Enums vs. Numeric Enums

The enums we have studied so far are referred to as numeric enums, since they are based on numbers. TypeScript also allows us to use enums based on strings, referred to as string enums. They are defined very similarly:

enum DirectionNumber { North, South, East, West }
enum DirectionString { North = 'NORTH', South = 'SOUTH', East = 'EAST', West = 'WEST' }

With numeric enums, the numbers could be assigned automatically, but with string enums we must write the string explicitly, as shown above. Technically, any string will do: North = 'JabberWocky' is a valid value definition. However, it is much better to use the convention shown here (North = 'NORTH'), where the string value of the enum variable is just the capitalized form of the variable name. This way, error messages and logs will be much more informative.

We recommend to always use string enums because numeric enums allow for some behaviors that can let bugs sneak into our code. For example, numbers can be assigned directly to numeric enum variables:

let whichWayToAntarctica: DirectionNumber;
whichWayToAntarctica = 1; // Valid TypeScript code.
whichWayToAntarctica = DirectionNumber.South; // Valid, equivalent to the above line.

Strangely, even assigning arbitrary numbers, as in whichWayToAntarctica = 943205, will not lead to type errors. String enums are much more strict. With string enums, variables cannot be assigned to strings at all!

let whichWayToAntarctica: DirectionString;
whichWayToAntarctica = '\ (•◡•) / Arbitrary String \ (•◡•) /'; // Type error!
whichWayToAntarctica = 'SOUTH'; // STILL a type error!
whichWayToAntarctica = DirectionString.South; // The only allowable way to do this.