deepMapKeys

Deep maps an object's keys.

• Creates an object with the same values as the provided object and keys generated by running the provided function for each key.
• Use Object.keys(obj) to iterate over the object's keys.
• Use Array.prototype.reduce() to create a new object with the same values and mapped keys using fn.

const deepMapKeys = (obj, fn) =>
  Array.isArray(obj)
    ? obj.map(val => deepMapKeys(val, fn))
    : typeof obj === 'object'
    ? Object.keys(obj).reduce((acc, current) => {
        const key = fn(current);
        const val = obj[current];
        acc[key] =
          val !== null && typeof val === 'object' ? deepMapKeys(val, fn) : val;
        return acc;
      }, {})
    : obj;

const obj = {
  foo: '1',
  nested: {
    child: {
      withArray: [
        {
          grandChild: ['hello']
        }
      ]
    }
  }
};
const upperKeysObj = deepMapKeys(obj, key => key.toUpperCase());
/*
{
  "FOO":"1",
  "NESTED":{
    "CHILD":{
      "WITHARRAY":[
        {
          "GRANDCHILD":[ 'hello' ]
        }
      ]
    }
  }
}
*/