unless

Returns the Maybe<T> value if the supplied predicate is false, otherwise null. This method supports chaining, and is used to conditionally run the rest of the chained methods.

import { maybe } from "@tsly/maybe";
 
function foo(arg?: number[]): string {
    return maybe(arg)
        ?.unless(it => it.length == 0)
        ?.take(it => it.reduce((prev, cur) => prev + cur))
        .toString() ?? "invalid";
}
 
console.log(foo()); // "invalid"
console.log(foo([])); // "invalid"
console.log(foo([1, 2, 3])); // "6"

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if try