Undefined to Null

Undefined to Null - Problem

JavaScript Medium

Given a deeply nested object or array obj, return the object obj with any undefined values replaced by null.

Why this matters: 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.

Note: The function should handle arbitrarily deep nesting of objects and arrays.

Input & Output

Example 1 — Basic Object

$ Input: obj = {"a": 1, "b": null, "c": undefined}

› Output: {"a": 1, "b": null, "c": null}

💡 Note: The undefined value at key 'c' is replaced with null. The existing null value at key 'b' remains unchanged.

Example 2 — Nested Array

$ Input: obj = [1, undefined, [2, undefined, 3]]

› Output: [1, null, [2, null, 3]]

💡 Note: Both undefined values are replaced with null - one at index 1 of outer array and one at index 1 of inner array.

Example 3 — Mixed Nesting

$ Input: obj = {"arr": [undefined, {"nested": undefined}]}

› Output: {"arr": [null, {"nested": null}]}

💡 Note: All undefined values are replaced regardless of nesting depth - in the array element and in the nested object property.

Constraints

The input can be any valid JavaScript value
Objects can be nested to arbitrary depth
Arrays can contain mixed types including objects
undefined values can appear at any nesting level

Visualization

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Asked in

G Google 25 f Facebook 20 a Amazon 15

The key insight is to recursively traverse the nested structure and replace undefined values with null. Best approach is recursive traversal with type checking. Time: O(n), Space: O(d)

Common Approaches

✓ In-Place Modification

⏱️ Time: O(n) Space: O(d)

Instead of creating new objects and arrays, modify the original structure in-place. This reduces memory usage and is more efficient for large nested structures.

Recursive Deep Traversal

⏱️ Time: O(n) Space: O(d)

Check the type of the input - if it's an object, iterate through all properties; if it's an array, iterate through all elements. For each value found, recursively process it and replace any undefined values with null.

In-Place Modification — Algorithm Steps

Check if input is array or object
For objects: iterate keys and modify values in-place
For arrays: iterate indices and modify elements in-place
Recursively process nested structures

Visualization

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Step-by-Step Walkthrough

Access Original

Work directly with the input object

Modify In-Place

Change undefined values to null directly

Return Modified

Return the same object reference

Code -

solution.c — C

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

char* solution(char* obj) {
    char* pos = obj;
    while ((pos = strstr(pos, "undefined")) != NULL) {
        memmove(pos + 4, pos + 9, strlen(pos + 9) + 1);
        memcpy(pos, "null", 4);
        pos += 4;
    }
    return obj;
}

int main() {
    char* obj = malloc(10000);
    fgets(obj, 10000, stdin);
    obj[strcspn(obj, "\n")] = 0;
    
    char* result = solution(obj);
    printf("%s\n", result);
    free(obj);
    return 0;
}

Time & Space Complexity

Time Complexity

⏱️

O(n)

Still need to visit every element once

✓ Linear Growth

Space Complexity

O(d)

Only recursion stack space needed, no extra object creation

✓ Linear Space

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Smart Actions

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Input & Output

Constraints

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Related Problems

Common Approaches

In-Place Modification — Algorithm Steps

Visualization

Code -

Time & Space Complexity

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