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2775. Undefined to Null


Write a function called undefinedToNull that takes a deeply nested object or array obj, and then creates a copy of that object with any undefined values replaced by null.

undefined values are handled differently than null values when objects are converted to a JSON string using JSON.stringify(). This function helps ensure serialized data is free of unexpected errors.


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



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


  • function undefinedToNull(obj: Record<any, any>): Record<any, any> {
        for (const key in obj) {
            if (typeof obj[key] === 'object') {
                obj[key] = undefinedToNull(obj[key]);
            if (obj[key] === undefined) {
                obj[key] = null;
        return obj;
     * undefinedToNull({"a": undefined, "b": 3}) // {"a": null, "b": 3}
     * undefinedToNull([undefined, undefined]) // [null, null]

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