Since the typescript now supports conditional types, I've decided to do some meta-programing to add more flavor to VSCODE intellisense. However, while other types are easy to separate using A extends B I have a hard time determining if the provided type is literal.
So the question would be - how do I determine if given type is of literal type?
I'm not sure what your use cases are. Personally, I would do something like this:
type IfStringOrNumberLiteral<T, Y=true, N=false> =
string extends T ? N : // must be narrower than string
number extends T ? N : // must be narrower than number
[T] extends [never] ? N : // must be wider than never
[T] extends [string | number] ? Y : // must be narrower than string | number
N
I always use --strictNullChecks so your mileage may vary when it comes to how that treats null and undefined. Of course it can be amended to meet any particular need you have. Mostly I just wanted to show an alternative to circuitous constructs of the form ( X extends Y ? true : false ) extends true ? U : V.
Hope that helps; good luck.
[...] are for cosmetic purpose only?
T. Since T is what's called a "naked type parameter", the phrase T extends string | number ? Y : N could end up becoming Y | N if T is a union like 4 | object, and I think you just want N in that case. The way to prevent distributive conditional types it is to "clothe" the naked type parameter in some (covariant) way. The tersest way to do that is the one-tuple bracket syntax.Edit:
I've rewritten everything to match jcalz's clean style:
type IsStringLiteral<T> =
string extends T ? false : // must be narrower than string
[T] extends [never | undefined | null] ? false : // must be wider than never and nullable
[T] extends [string] ? true : // must be wider than string
false;
type IsNumberLiteral<T> =
number extends T ? false : // must be narrower than number
[T] extends [never | undefined | null] ? false : // must be wider than never and nullable
[T] extends [number] ? true : // must be wider than number
false;
type IsSingleTypeLiteral<T> =
IsStringLiteral<T> extends false ?
IsNumberLiteral<T> :
true;
type IsLiteral<T> =
string extends T ? false : // must be narrower than string
number extends T ? false : // must be narrower than number
[T] extends [never | undefined | null] ? false : // must be wider than never and nullable
[T] extends [number | string] ? true : // must be wider than number | string
false;
That was a little harder than anticipated, but after few hours I managed to achieve this:
type Switch<A, B, IF, ELSE = A> = A extends B ? IF : ELSE;
type IsStringLiteral<T> =
// Check for nullable type using Switch type. See next comment why Switch must be used.
Switch<T, undefined | null, true, false> extends true ? false : (
// `T extends string` does not work for `"str" | number` and etc. Results in `boolean` type.
// Need to use boolean Switch to filter out false-positive.
Switch<T, string, true, false> extends true ? (
// `string` does not extend literal type.
string extends T ? false : true
) : false
);
type IsNumberLiteral<T> =
Switch<T, undefined | null, true, false> extends true ? false : (
Switch<T, number, true, false> extends true ? (
number extends T ? false : true
) : false
);
type IsSingleTypeLiteral<T> =
Switch<IsStringLiteral<T>, false, IsNumberLiteral<T>, true>;
type IsLiteral<T> =
// `"string literal" | string` and etc. will return a false-positive `boolean` type.
// `boolean` type must always be `false`, thus `false extends boolean` is used to get that `false` type.
Switch<false, Switch<T, undefined | null, true, false> extends true ? false : (
T extends string | number ? (
string extends T ? false : (number extends T ? false : true)
) : false
), false, true>;
Here are some test cases in form of HTML table (tested with 3.1.1):
table, th, td {
white-space: nowrap;
border: 1px solid black;
}<table><tbody><tr><th>Test cases</th><th>IsStringLiteral</th><th>IsNumberLiteral</th><th>IsSingleTypeLiteral</th><th>IsLiteral</th></tr><tr><td>"string literal"</td><td><b>true</b></td><td>false</td><td><b>true</b></td><td><b>true</b></td></tr><tr><td>123</td><td>false</td><td><b>true</b></td><td><b>true</b></td><td><b>true</b></td></tr><tr><td>string</td><td>false</td><td>false</td><td>false</td><td>false</td></tr><tr><td>object</td><td>false</td><td>false</td><td>false</td><td>false</td></tr><tr><td>[]</td><td>false</td><td>false</td><td>false</td><td>false</td></tr><tr><td>[string, number]</td><td>false</td><td>false</td><td>false</td><td>false</td></tr><tr><td>any</td><td>false</td><td>false</td><td>false</td><td>false</td></tr><tr><td>void</td><td>false</td><td>false</td><td>false</td><td>false</td></tr><tr><td>null</td><td>false</td><td>false</td><td>false</td><td>false</td></tr><tr><td>undefined</td><td>false</td><td>false</td><td>false</td><td>false</td></tr><tr><td>never</td><td>false</td><td>false</td><td>false</td><td>false</td></tr><tr><td>"string literal" | 123</td><td>false</td><td>false</td><td>false</td><td><b>true</b></td></tr><tr><td>"string literal" | string</td><td>false</td><td>false</td><td>false</td><td>false</td></tr><tr><td>123 | number</td><td>false</td><td>false</td><td>false</td><td>false</td></tr></tbody></table>
"a" | "b"to be of literal type? What about2 | string?"a" | "b",1 | 2and maybe"a" | 1to be of literal type. Any other union is invalid for me.