AI solution for JavaScript Coding Question on BFE.dev
176. undefined to null

Here's a possible solution:

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;
  }
}

The function checks if the input is an array, an object or a primitive value, and treats each case accordingly. If it's an array, it maps over each element, recursively calling undefinedToNull on them. If it's an object, it creates a new object and iterates over its properties, recursively calling undefinedToNull on each value. If it's a primitive value, it returns either null if the value is undefined, or the value itself otherwise.

Note that this implementation assumes that the input is a valid JSON object or array (i.e. no circular references, no functions, no symbol properties, etc.). If the input is a more general object, it might need additional checks or custom handling.