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Variables, Types, and Collections23% of the exam

JSON parse and stringify

Turning JSON text into objects and back: strict JSON syntax, navigating nested response data, the JSON.parse reviver, what JSON.stringify drops or converts (undefined, functions, Date, Map, NaN), replacer arrays and functions, indentation, toJSON, and the limits of a JSON round-trip deep copy.

Official exam objective
Given a JSON response, demonstrate how to operate the JSON object.

~5 min read · 10 practice questions ·

What the exam expects

  • Recognise valid JSON text and predict when JSON.parse() throws SyntaxError.
  • Parse a response and read nested values with dot, bracket and array-index access, iterating arrays with for...of or array methods.
  • Transform values while parsing with a reviver (for example, ISO strings to Date).
  • Predict JSON.stringify() output for undefined, functions, null, NaN, Date, Map, Set and objects with toJSON().
  • Filter or reshape output with a replacer array or function, and indent it with the space argument.
  • Explain what a JSON.parse(JSON.stringify(x)) copy loses.
  • Key terms: JSON text (a string in JSON syntax, not an object); parse (text → JavaScript value); serialize/stringify (value → JSON text); reviver (a parse callback that can replace each value); replacer (a stringify array or callback that selects or replaces values).

Key facts

JSON property names
Detail
Double-quoted strings only; no trailing commas
Source
JSON (opens in a new tab)
Invalid JSON text
Detail
JSON.parse() throws SyntaxError
Source
JSON.parse() (opens in a new tab)
Reviver returns undefined
Detail
That property is deleted (the root too)
Source
JSON.parse() (opens in a new tab)
undefined/function in an object
Detail
Omitted
Source
JSON.stringify() (opens in a new tab)
undefined/function in an array
Detail
Becomes null
Source
JSON.stringify() (opens in a new tab)
NaN, Infinity
Detail
Serialized as null
Source
JSON.stringify() (opens in a new tab)
Date
Detail
ISO string from toJSON()
Source
Date.prototype.toJSON() (opens in a new tab)
Map, Set
Detail
{}
Source
JSON.stringify() (opens in a new tab)
BigInt or circular reference
Detail
TypeError
Source
JSON.stringify() (opens in a new tab)
Replacer array
Detail
Allowlist of property names
Source
JSON.stringify() (opens in a new tab)
Replacer function first call
Detail
Key "", value = the root object
Source
JSON.stringify() (opens in a new tab)
space argument
Detail
Indentation only; capped at 10
Source
JSON.stringify() (opens in a new tab)
Sources for this section (4)

Check yourself

  • Why does `JSON.parse("{'a': 1}")` throw?
  • When should you use a reviver instead of post-processing?
  • What does `JSON.stringify({ tags: ['a', undefined], note: undefined })` return?
  • Why does a replacer function that returns `undefined` for every key except `password` give `undefined`?
  • What type is a `Date` property after `JSON.parse(JSON.stringify(obj))`?

The questions are free to read. Answers with their sources and self-grading are included with the pack.

Keep reading with the JavaScript Developer pack

  • Core concepts
  • Exam traps
  • Worked scenario

The pack includes:

  • All 24 lessons of the JavaScript Developer guide, with reading progress
  • Interactive decision walks and step-by-step sequence steppers
  • Inline practice questions and one-click topic practice (10 questions for this lesson)
  • An official source cited for every claim
  • An AI tutor that answers from the guide
  • Unlimited timed practice exams
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