const declarations

const declarations

const declarations are another way of declaring variables.

const numLivesForCat = 9;

They are like let declarations but, as their name implies, their value cannot be changed once they are bound. In other words, they have the same scoping rules as let, but you can’t re-assign to them.

This should not be confused with the idea that the values they refer to are immutable.

const numLivesForCat = 9;
const kitty = {
  name: "Aurora",
  numLives: numLivesForCat,
}

// Error
kitty = {
  name: "Danielle",
  numLives: numLivesForCat
};

// all "okay"
kitty.name = "Rory";
kitty.name = "Kitty";
kitty.name = "Cat";
kitty.numLives--;

Unless you take specific measures to avoid it, the internal state of a const variable is still modifiable. Fortunately, TypeScript allows you to specify that members of an object are readonly. The chapter on Interfaces has the details.