Enums is not a type-level extension of JavaScript. It allow a developer to define a set of named constants. Using enums can make it easier to document intent, or create a set of distinct cases. TypeScript provides both numeric and string-based enums.

Numeric Enum

Here is an example of a numeric enum in TypeScript:

enum Direction {
  Up = 1,
  Down,
  Left,
  Right,
}

Above, we have a numeric enum where Up is initialized with 1. All of the following members are auto-incremented from that point on. In other words, Direction.Up has the value 1, Down has 2, Left has 3, and Right has 4.

If we left off the initializer for Up, it would have the value 0 and the rest of the members would be auto-incremented from there.

String-based Enum

In a string-based enum, each member has to be initialized.

enum Direction {
  Up = "UP",
  Down = "DOWN",
  Left = "LEFT",
  Right = "RIGHT",
}

Why enum?

enum injects a JavaScript runtime object into your code, but with a constant constrain. For example

enum Test {
    name = 'Andrew',
    age = 0
}

const obj = {
    name: "Andrew",
    age: 0
}

Test.name = "ha"; // Cannot assign to 'name' because it is a read-only property.

obj.name = "ha"; // no problem

relatively, if you want to equip your object with constant property, use as const assertion to make your object members be read-only