Common JSON Mistake. Why JSON.stringify Ignores Undefined but Keeps Null JavaScript Deep Dive. Why Undefined Disappears in JSON.stringify but Null Stays

Lets explain the Code:

let x = {
    a: undefined,
    b: null
}
console.log(JSON.stringify(x));

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Step 1: Object Declaration

let x = {
    a: undefined,
    b: null
}

What happens here:

• You are creating an object x with two properties:
• Property a is explicitly assigned undefined.
• Property b is explicitly assigned null.

Quick property breakdown:

• undefined

• Represents the absence of a value.
• Often appears when a variable or object property has been declared but not assigned a value.
• null
• Represents the intentional absence of any object value.
• It is explicitly set by the developer to indicate "no value" or "empty."

---

Step 2: JSON.stringify(x)

console.log(JSON.stringify(x));

What is JSON.stringify?

It is a built-in JavaScript method that converts a JavaScript object or value into a JSON string.

Key Rule:

• When you convert an object to JSON:

• Properties with undefined are excluded from the final string.
• Properties with null are included.

Why?

According to the JSON specification:

• JSON supports:
• Numbers
• Strings
• Booleans
• Arrays
• Objects
• null
• JSON does not support undefined as a valid value.

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How JSON.stringify Works in This Case:

Original object:

{
    a: undefined,
    b: null
}

After applying JSON.stringify(x):

'{"b":null}'

Explanation:

• Property a is ignored because its value is undefined (not serializable in JSON).
• Property b is included because null is a valid JSON value.

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✔ Final Answer:

B) {"b":null}

This question appeared in the Epam 2nd round of interview, testing the candidate’s knowledge of Json data type convert object into Json vise-versa in JavaScript.

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Why Does This Happen?

• JSON.stringify skips:
• undefined values
• Function properties
• Symbol properties

• JSON.stringify includes:

• Numbers
• Strings
• Booleans
• Objects
• Arrays
• null

---

Key Notes:

Value Type	Behavior in JSON.stringify
undefined	Skipped
null	Included
Function	Skipped
Symbol	Skipped

---

Example Extension:

let obj = {
    a: undefined,
    b: null,
    c: 42,
    d: "Hello"
}

console.log(JSON.stringify(obj));
// Output: {"b":null,"c":42,"d":"Hello"}

The undefined property is skipped, but null, numbers, and strings are included.

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Final Summary:

Step	Action	Result
1	Declare object with undefined and null	Object created
2	Apply JSON.stringify	undefined skipped
3	Print JSON string	{"b":null}

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Key Takeaways:

• undefined is not valid in JSON and is excluded from the JSON string.
• null is valid in JSON and is included.
• JSON.stringify creates cleaner JSON by removing non-serializable values automatically.