The implemented code does entirely skip null-values (thus, any path to a null-values gets omitted) and does not handle array-values properly (thus arrays are not getting "squashed"). But a squashObject function should cover such valid data-values as well.

A just slightly updated version of the code already does the trick …

function squashIt(source, path = '') {
  const squashed = {};

  for (let key in source) {

    const keypath = path ? `${ path }.${ key }` : key;
    const value = source[key];

    if (!!value && typeof value === 'object') {

      Object.assign(squashed, squashObject(value, keypath));
    } else {
      squashed[keypath] = value;
    }
  }
  return squashed;
}

… and enables the “squashing” of valid JSON-compatible data-structures …

const sampleData = {
  a: 1,
  b: {
    c: 2,
    d: {
      e: ['foo', {
        a: ['bar', 'baz', 'biz'],
          b: {
            c: null,
            d: 4,
          },
        }, 'buzz'],
      f: {
        g: null,
        h: 4,
      },
    },
  },
  i: 5,
};
const squashed = squashIt(sampleData);
console.log({ squashed });

… even though it still is not bulletproof.