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unless

unless() function​

If the result of predicate is true, process will not be executed. unless return the result of process if it is executed, and if the process is not executed, it returns the function argument as is.

Signature: 
declare function unless<T, U>(predicate: (input: T) => boolean, process: (input: T) => U): (input: T) => T | (U extends void ? undefined : U);

declare function unless<T, N extends T, U>(predicate: (input: T) => input is N, process: (input: Exclude<T, N>) => U): (input: T) => N | (U extends void ? undefined : U);

original input or result of process

Example​

// it will return only string
const unlessIsString: (input: string | undefined) => string = unless(isString, (input) => {
   throw Error("input is undefiend.")
});

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