https://leetcode.com/problems/first-missing-positive/

firstMissingPositive.js

const firstMissingPositive =
    xs => {
        for ( const x of xs )
            indexByValue( x , xs )
        let ndx = 0
        while ( xs[ ndx ] === ++ndx ) {}
        return ndx
    }
        
const indexByValue =
    ( x , xs ) =>
        xs[ x - 1 ] !== x
        && 0 < x
        && x <= xs.length
        && ( y = xs[ x - 1 ]
           , xs[ x - 1 ] = x
           , indexByValue( y , xs )
           )