Undefined to Null

Problem Statement

Leveraging data stored in JSON objects or arrays requires null-safe operations, especially when encoding using JSON.stringify(). Given this context, the task involves taking a nested JSON object or array, denoted as obj, and replacing all occurrences where the value is undefined with null. This manipulation ensures that the serialization of obj into a JSON string doesn't lead to errors or unintended results, as undefined and null are treated differently during serialization: undefined fields are omitted, whereas null fields are retained with the value null.

Examples

Example 1

Input:

obj = {"a": undefined, "b": 3}

Output:

{"a": null, "b": 3}

Explanation:

The value for obj.a has been changed from undefined to null

Example 2

Input:

obj = {"a": undefined, "b": ["a", undefined]}

Output:

{"a": null,"b": ["a", null]}

Explanation:

The values for obj.a and obj.b[1] have been changed from undefined to null

Constraints

• obj is a valid JSON object or array
• 2 <= JSON.stringify(obj).length <= 105

Approach and Intuition

To tackle the problem of replacing undefined values in a nested structure with null, one needs to understand the nature of undefined in JavaScript and its implications during JSON serialization:

1. Recursive Traversal of Object/Array:

   • If obj is an object, loop through each property.
   • If obj is an array, iterate over each element.

2. Value Replacement:

   • Check if the property's or element's value is undefined.
   • If it is undefined, replace it with null.

3. Handling Nested Structures:

   • If an encountered property or element is an object or an array itself, recursively apply the same process.

Example Walkthroughs for Clarity

• Example 1:

  • Input is {"a": undefined, "b": 3}.
  • The property "a" is identified as undefined and replaced with null, resulting in {"a": null, "b": 3}.

• Example 2:

  • Here, both a property and an element within an array are undefined.
  • The process replaces undefined with null both for property "a" and the second element of array under property "b", leading to {"a": null,"b": ["a", null]}.

Understanding and implementing this approach efficiently ensures that the JSON serialized output is predictable and correctly formatted, especially in contexts where data integrity and format are crucial, such as API responses or data storage operations.

Solutions

/**
 * Function to convert undefined values to null within an object or array.
 * @param {Object|Array} data - The object or array to process.
 * @return {Object|Array} - The modified object or array with undefined values replaced by null.
 */
var replaceUndefinedWithNull = function(data) {
    if (typeof data !== 'object' || data === null) {
        return data !== undefined ? data : null;
    }
    
    if (Array.isArray(data)) {
        return data.map(element => replaceUndefinedWithNull(element));
    }
        
    const output = {};
        
    for (const key in data) {
        output[key] = replaceUndefinedWithNull(data[key]);
    }
    
    return output;
};