176. undefined to null

You are asked to implement undefinedToNull() to return a copy that has all undefined replaced with null.

>undefinedToNull({a: undefined, b: 'BFE.dev'})
>// {a: null, b: 'BFE.dev'}

>undefinedToNull({a: ['BFE.dev', undefined, 'bigfrontend.dev']})
>// {a: ['BFE.dev', null, 'bigfrontend.dev']}

Answer

function undefinedToNull(obj: any): any {
  if (Array.isArray(obj)) {
    return obj.map(elem => undefinedToNull(elem));
  } else if (typeof obj === 'object' && obj !== null) {
    const newObj = {};
    for (const key in obj) {
      if (Object.prototype.hasOwnProperty.call(obj, key)) {
        newObj[key] = undefinedToNull(obj[key]);
      }
    }
    return newObj;
  } else {
    return obj === undefined ? null : obj;
  }
}