// Global permutations array for ease-of-use by functions.
let permutations = [];

// Separate from getPermutations because this part gets recursively called.
const getPermutationsInner = ( items, wipPermutation = [] ) => {

    items.forEach( ( item, index ) => {

        let wipPerm = wipPermutation.concat( item );

        // Clone items via slice().
        const otherItems = items.slice();

        // splice() modifies arrays, unlike slice().
        otherItems.splice( index, 1 );

        if ( otherItems.length === 0 )
            permutations.push( wipPerm );
        else
            getPermutationsInner( otherItems, wipPerm );

    } );

};

// Gets all permutations of an array of items. Assumes every item is unique.
const getPermutations = items => {

    getPermutationsInner( items );

    // Clone current value of permutations.
    const returnedPermutations = permutations;

    // Reset permutations variable so it is empty when the function is called again.
    permutations = [];

    return returnedPermutations;

};

// Gets all permutations of the characters of a string. Assumes every character is unique.
const getPermutationsOfString = str =>
    getPermutations( str.split( '' ) ).map( permutation => permutation.join( '' ) );

const permAlone = str => {

    // Get all possible permutations of the string.
    const permutations = getPermutationsOfString( str );

    // Filter out all permutations that have 2 or more identical consecutive characters.
    const permutationsWithoutRepeatedChars = permutations.filter(
        permutation => permutation.match( /(.)(?=\1)/g ) === null ? true : false
    );

    return permutationsWithoutRepeatedChars.length;

};

permAlone( 'aab' );