Undefined to Null - Practice Coding Problems

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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Understanding the Visualization

Input

Nested object/array with undefined values

Process

Recursively find and replace undefined with null

Output

Clean structure with null values

Key Takeaway

🎯 Key Insight: Recursively traverse nested structures and replace undefined with null to ensure safe JSON serialization

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

Approach	Time	Space	Notes
✓ Recursive Deep Traversal	O(n)	O(d)	Recursively traverse every property and array element, replacing undefined with null
In-Place Modification	O(n)	O(d)	Modify the original object in-place to avoid creating new objects

Recursive Deep Traversal — Algorithm Steps

Check if input is array, object, or primitive
For objects: iterate through all keys and recursively process values
For arrays: iterate through all indices and recursively process elements
Replace undefined values with null, return other values as-is

Visualization

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Step-by-Step Walkthrough

Check Type

Determine if input is array, object, or primitive

Process Recursively

For each nested element, apply the same logic

Replace undefined

Convert any undefined values to null

Code -

solution.c — C

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

char* solution(char* obj) {
    int len = strlen(obj);
    char* result = malloc(len * 2);
    strcpy(result, obj);
    
    char* pos = result;
    while ((pos = strstr(pos, "undefined")) != NULL) {
        memmove(pos + 4, pos + 9, strlen(pos + 9) + 1);
        memcpy(pos, "null", 4);
        pos += 4;
    }
    
    return result;
}

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

Time & Space Complexity

Time Complexity

⏱️

O(n)

Visit each element/property exactly once where n is total number of values

✓ Linear Growth

Space Complexity

O(d)

Recursion stack depth d equals maximum nesting level

✓ Linear Space

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Smart Actions

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Input & Output

Constraints

Visualization

Related Problems

Common Approaches

Recursive Deep Traversal — Algorithm Steps

Visualization

Code -

Time & Space Complexity

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