flatten an object into a single depth using string keys

flatten_and_test.js

require('tap-browser-color')();
var test = require('tape');

test('it flattens!', function(t) {
  var input = {
    users: [
      { name: 'name1', id: 1, image: { '64x64': 'http://1' } },
      { name: 'name2', id: 2, image: { '64x64': 'http://2' } }
    ],
    errors: [ new Error('err1') ],
    success: true
  }
  
  var expected = {
    'users.0.name': 'name1',
    'users.0.id': 1,
    'users.0.image.64x64': 'http://1',
    'users.1.name': 'name2',
    'users.1.id': 2,
    'users.1.image.64x64': 'http://2',
    'users.length': 2,
    'errors.0.columnNumber': input.errors[0].columnNumber,
    'errors.0.fileName': input.errors[0].fileName,
    'errors.0.lineNumber': input.errors[0].lineNumber,
    'errors.0.message': input.errors[0].message,
    'errors.0.stack': input.errors[0].stack,
    'errors.length': 1,
    'success': true
  }

  t.deepEqual(flatten(input), expected);
  t.end();
})


function flatten(obj, opt_out, opt_paths) {
  var out = opt_out || {};
  var paths = opt_paths || [];
  return Object.getOwnPropertyNames(obj).reduce(function(out, key) {
    paths.push(key);
    if (typeof obj[key] === 'object') {
      flatten(obj[key], out, paths);  
    } else {
      out[paths.join('.')] = obj[key];
    }
    paths.pop();
    return out;
  }, out)
}

Here's one that doesn't requiring mutating an array. I was asked this in an phone screen today (and bombed the implementation):

const pathDelimiter = '.'

function flatten(source, flattened = {}, keySoFar = '') {
  function getNextKey(key) {
    return `${keySoFar}${keySoFar ? pathDelimiter : ''}${key}`
  }
  if (typeof source === 'object') {
    for (const key in source) {
      flatten(source[key], flattened, getNextKey(key))
    }
  } else {
    flattened[keySoFar] = source
  }
  return flattened
}