Flatten An Arbitrarily Nested Numerical Array

flatten.js

function flatten( array ) {
    const output = [];

    if( !( array instanceof Array ) )
        return false;

    function recurseFlatten( array ) {
        array.forEach( (el) => {
        
        if( el instanceof Array ) {
            recurseFlatten( el );
        } else if ( typeof el === 'number' ) {
            output.push( el );
        }
    });
    }

    recurseFlatten( array );

    return output;
}