Enums

ℹ️ Used as a collection of related values. Use enums when a method or variable can only take one out of a small set of possible values.

An enum is a way to organize a collection of related values.

enum Seasons {
  Summer,
  Autumn,
  Winter,
  Spring,
}

These enums values are number-based:

const summer = Seasons.Summer;
console.log(summer);
// -> 0

Direction.Up is 0, Direction.Down is 1 and so on...

Now, let's look at the JavaScript that it generates:

var Seasons;
(function(Seasons) {
  Seasons[(Seasons['Summer'] = 0)] = 'Summer';
  Seasons[(Seasons['Autumn'] = 1)] = 'Autumn';
  Seasons[(Seasons['Winter'] = 2)] = 'Winter';
  Seasons[(Seasons['Spring'] = 3)] = 'Spring';
})(Seasons || (Seasons = {}));

Let's take the line Seasons[Seasons["Summer"] = 0] = "Summer";. To understand this line we just need to analyze Seasons["Summer"] = 0, here we can see that it assigns value 0 to the Seasons.Summer, then we know that assignment operator returns the assigned value, so further execution is Seasons[0] = "Summer".

console.log(Seasons.Summer); // 0
console.log(Seasons[0]); // "Summer"

Enums have autoincrementing behavior:

enum Seasons {
  Summer = 1,
  Autumn,
  Winter,
  Spring,
}

When Up is initilized with 1, others have auto-incremented value from that point. Direction.Down has value 2, Direction.Left has 3, Direction.Right has 4.

String Enums

enum Seasons {
  Summer = 'SUMMER',
  Autumn = 'AUTUMN',
  Winter = 'WINTER',
  Spring = 'SPRING',
}

String enums don't have auto-increment behavior. It's required to give assign a value to each enum member.

Generated code is the following:

var Season;
(function(Season) {
  Season['Summer'] = 'SUMMER';
  Season['Autumn'] = 'AUTUMN';
  Season['Winter'] = 'WINTER';
  Season['Spring'] = 'SPRING';
})(Season || (Season = {}));

So

console.log(Season.Summer); // -> "SUMMER"
console.log(Season[0]); // -> undefined

Read More

• TypeScript handbook - Enums