class Coordinate {
constructor(readonly x: number, readonly y: number) {}
}
const Up = new Coordinate(0, -1);
const Right = new Coordinate(1, 0);
const Down = new Coordinate(0, 1);
const Left = new Coordinate(-1, 0);
// How can we restrict the direction parameter to only accept values of Up, Right, Down, or Left?
function move(direction: Coordinate) {
...
}
Is there a way to ensure that the direction parameter is limited to either Up, Right, Down, or Left?
If you want to constrain a TypeScript function input to specific values, you must define those values at the type level using "literal" types. These types only accept particular values, allowing you to create restrictions by combining these literal types as unions.
Within TypeScript, primitive literal types exist for strings like "hello", numbers like 123, boolean values like true, and special types such as null and undefined. Additionally, there are unique symbols and enums for numeric and string literals.
Unfortunately, TypeScript lacks literal object types, preventing the definition of specific objects as distinct types. Using generics can provide a solution by making properties generic, enabling distinctions between approved values at the type level.
An alternative method is to utilize literal types like string enums to designate approved objects through key-value mappings. By leveraging an enum akin to Direction, you can map each direction to its associated coordinates and ensure that the move() function accepts only these predefined values.
Ultimately, depending on your requirements, you can adopt either approach to restrict function inputs based on specific values in TypeScript.
One approach you could take is outlined below, although it comes with some limitations:
class Coordinate<X extends number, Y extends number> {
constructor(
readonly x: X,
readonly y: Y
) {}
}
const Up = new Coordinate(0, -1);
const Right = new Coordinate(1, 0);
const Down = new Coordinate(0, 1);
const Left = new Coordinate(-1, 0);
type Direction =
typeof Up |
typeof Right |
typeof Down |
typeof Left;
function move(direction: Direction) {
// ...
}
move(Up);
move(new Coordinate(1, 0));
move(new Coordinate(1, 1)); // Error
direction and the constants won't work as it compares instances, not values.To address these limitations, consider this alternative approach:
class Coordinate {
readonly #key = {}
private constructor(
readonly x: number,
readonly y: number
) {}
static Up = new Coordinate(0, -1);
static Right = new Coordinate(1, 0);
static Down = new Coordinate(0, 1);
static Left = new Coordinate(-1, 0);
}
const { Up, Right, Down, Left } = Coordinate;
function move(direction: Coordinate) {
// ...
}
move(Up);
move({ x: 1, y: 0 }); // Error (Property '#key' is missing...)
move(new Coordinate(1, 0)); // Error (Constructor of class 'Coordinate' is private)
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