IsStringLiteral<T>

IsStringLiteral<T> = IfNever<T, false, T extends string ? { } extends Record<T, never> ? false : true : false>

Defined in: packages/react-querybuilder/src/types/type-fest/is-literal.ts:116

Returns a boolean for whether the given type is a string literal type.

Useful for:

• providing strongly-typed string manipulation functions
• constraining strings to be a string literal
• type utilities, such as when constructing parsers and ASTs

The implementation of this type is inspired by the trick mentioned in this StackOverflow answer.

Type Parameters

Type Parameter
T

Examples

import type {IsStringLiteral} from 'type-fest';

type CapitalizedString<T extends string> = IsStringLiteral<T> extends true ? Capitalize<T> : string;

// https://github.com/yankeeinlondon/native-dash/blob/master/src/capitalize.ts
function capitalize<T extends Readonly<string>>(input: T): CapitalizedString<T> {
	return (input.slice(0, 1).toUpperCase() + input.slice(1)) as CapitalizedString<T>;
}

const output = capitalize('hello, world!');
//=> 'Hello, world!'

// String types with infinite set of possible values return `false`.

import type {IsStringLiteral} from 'type-fest';

type AllUppercaseStrings = IsStringLiteral<Uppercase<string>>;
//=> false

type StringsStartingWithOn = IsStringLiteral<`on${string}`>;
//=> false

// This behaviour is particularly useful in string manipulation utilities, as infinite string types often require separate handling.

type Length<S extends string, Counter extends never[] = []> =
	IsStringLiteral<S> extends false
		? number // return `number` for infinite string types
		: S extends `${string}${infer Tail}`
			? Length<Tail, [...Counter, never]>
			: Counter['length'];

type L1 = Length<Lowercase<string>>;
//=> number

type L2 = Length<`${number}`>;
//=> number

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caution

API documentation is generated from the latest commit on the main branch. It may be somewhat inconsistent with official releases of React Query Builder.