make accumulator an array js code example

Example 1: reduce array to object javascript

const posts = [
    {id: 1, category: "frontend", title: "All About That Sass"},
    {id: 2, category: "backend", title: "Beam me up, Scotty: Apache Beam tips"},
    {id: 3, category: "frontend", title: "Sanitizing HTML: Going antibactirial on XSS attacks"}
];

const categoryPosts = posts.reduce((acc, post) => {
    let {id, category} = post;
    return {...acc, [category]: [...(acc[category] || []), id]};
}, {});

Example 2: array reduce

arr.reduce(callback( accumulator, currentValue[, index[, array]] ) {
  // return result from executing something for accumulator or currentValue
}[, initialValue]);