index.js

app.get('/characters', function (req, res) {
    let mappedCharacters = [];

    function mapResults(json){
        mappedCharacters.push(json);
        return mappedCharacters;
    }

    function reducePages(pages){
        return pages.reduce((acc, curr) =>
            acc.concat(curr), []
        );
    }

    function getAllCharacters(pageNum){
        let url = `${swapi}/people/?page=${pageNum}`;

        return fetch(url)
            .then(res => res.json())
            .then(json => json.results)
    }

    const allPages = [1,2,3,4,5].map((page) => getAllCharacters(page)
        .then(json => mapResults(json))
        .then(pages => reducePages(pages))
    );

    const allCharacters = Promise.all(allPages).then(response => response);

    console.log(allCharacters);

    res.render('characters', {characters: JSON.stringify(allCharacters)})
})