Description
Allows a String to be used as a literal type.
Syntax
let foo: 'Hello';
foo = 'Bar'; // Error: "Bar" is not assignable to type "Hello"Examples
Unions
This is useful when uses with Unions:
type CardinalDirection =
| "North"
| "East"
| "South"
| "West";
function move(distance: number, direction: CardinalDirection) {
// ...
}
move(1,"North"); // Okay
move(1,"Nurth"); // Error!Enums
If you want to go nuts you can mock a string based Enum:
/** Utility function to create a K:V from a list of strings */
function strEnum<T extends string>(o: Array<T>): {[K in T]: K} {
return o.reduce((res, key) => {
res[key] = key;
return res;
}, Object.create(null));
}
/**
* Sample create a string enum
*/
/** Create a K:V */
const Direction = strEnum([
'North',
'South',
'East',
'West'
])
/** Create a Type */
type Direction = keyof typeof Direction;
/**
* Sample using a string enum
*/
let sample: Direction;
sample = Direction.North; // Okay
sample = 'North'; // Okay
sample = 'AnythingElse'; // ERROR!Property names
String types can be uses as property names in mapped types.
type Options = {
[K in "noImplicitAny" | "strictNullChecks" | "strictFunctionTypes"]?: boolean
};
// same as
// type Options = {
// noImplicitAny?: boolean,
// strictNullChecks?: boolean,
// strictFunctionTypes?: boolean
// };