TypeScript also supports boolean and number literal types, e.g.:
type OneToFive = 1 | 2 | 3 | 4 | 5;
type Bools = true | false;
Inference
Quite commonly you get an error like Type string is not assignable to type "foo". The following example demonstrates this.
function iTakeFoo(foo: 'foo') { }
const test = {
someProp: 'foo'
};
iTakeFoo(test.someProp); // Error: Argument of type string is not assignable to parameter of type 'foo'
This is because test is inferred to be of type {someProp: string}. The fix here is to use a simple type assertion to tell TypeScript the literal you want it to infer as shown below:
function iTakeFoo(foo: 'foo') { }
const test = {
someProp: 'foo' as 'foo'
};
iTakeFoo(test.someProp); // Okay!
or use a type annotation that helps TypeScript infer the correct thing at the point of declaration:
function iTakeFoo(foo: 'foo') { }
type Test = {
someProp: 'foo',
}
const test: Test = { // Annotate - inferred someProp is always === 'foo'
someProp: 'foo'
};
iTakeFoo(test.someProp); // Okay!
Use cases
Valid use cases for string literal types are:
String based enums
/** 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));
}
And then generate the literal type union using keyof typeof. Here is a complete example:
/** 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!
. You can use string literals with union types to mock a string based enum as we did in the CardinalDirection example above. You can even generate a Key:Value structure using the following function:
E.g. that can either be a boolean or the literal string "nocursor" (effective valid values true,false,"nocursor"). It can be declared as: