group-by-tag.js

let arr = [
  {
    title: 'Cat',
    tags: ['tag1', 'tag2'],
  },
  {
    title: 'Dog',
    tags: ['tag1', 'tag2'],
  },
  {
    title: 'Monkey',
    tags: ['tag1'],
  },
  {
    title: 'Squirrel',
    tags: ['tag1'],
  },
]

let groupedByTag = arr.reduce((acc, node) => {
  node.tags.forEach((tag) => {
    if (Array.isArray(acc[tag])) {
      acc[tag].push(node);
    } else {
      acc[tag] = [node];
    }
  });

  return acc;
}, {});

console.log(JSON.stringify(groupedByTag, null, 2));