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types: improve type inference for cx concatenation #64

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types: improve type inference for cx concatenation #64

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26 changes: 25 additions & 1 deletion src/index.ts
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Original file line number Diff line number Diff line change
@@ -1,11 +1,35 @@
export type ClassName = string | boolean | null | undefined;

type FilterStrings<T extends readonly unknown[]> = T extends readonly [
infer F,
...infer R,
]
? F extends string
? [F, ...FilterStrings<R>]
: FilterStrings<R>
: [];

type JoinStrings<T extends readonly string[]> = T extends readonly [
infer F,
...infer R,
]
? F extends string
? R extends readonly string[]
? R["length"] extends 0
? F
: `${F} ${JoinStrings<R>}`
: F
: never
: "";

/**
* Conditionally join classNames into a single string
* @param {...String} args The expressions to evaluate
* @returns {String} The joined classNames
*/
function cx(...args: ClassName[]): string;
function cx<T extends readonly ClassName[]>(
...args: T
): JoinStrings<FilterStrings<T>>;
function cx(): string {
let str = "",
i = 0,
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