After the code runs, why is `c.functionField` undefined?

const a = {
  stringField: 'Joe',
  nestedField: { field: 'Nested' },
  functionField: () => 'aReturn'
};
const b = Object.assign({}, a);
const c = JSON.parse(JSON.stringify(a));

console.log(b);
console.log(c);
console.log(typeof b.nestedField, typeof b.functionField);
console.log(typeof c.nestedField, typeof c.functionField);