getUniqueKeys.js

/**
 * Generates an array of unique keys from an array of objects
 * @param {Object[]} data The array to parse
 * @returns {string[]} The unique keys
 */
const getUniqueKeys = data => Array.from(
  new Set(
    data.reduce((allKeys, entry) => [...allKeys, ...Object.keys(entry)], [])
  ) 
);

const example = getUniqueKeys([
  { name: 'Misty', type: 'cat' },
  { name: 'Sam', age: 29 },
  { title: 'Eloquent Javascript', type: 'book' }
]); // ['name', 'type', 'age', 'title']