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1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errorsJSON (JavaScript Object Notation) is a text format for representing structured data so software can store it or exchange it with other systems. It is not a programming language, even though its syntax was derived from conventions used in JavaScript. A JSON document can contain strings, numbers, booleans, null, objects, or arrays.
{
"name": "Mina",
"active": true,
"score": 12,
"tags": ["blue", "green"],
"manager": null
}Here, curly braces enclose an object of named values, and square brackets enclose an ordered array. The example is valid JSON; what its fields mean depends on the application using it.
What JSON is used for
JSON is short for JavaScript Object Notation. The IETF’s RFC 8259 describes it as a text format for serializing structured data. A program can turn data into JSON text, send or save that text, and have another program parse it back into data. JSON is widely useful precisely because it is a format rather than a feature limited to one programming language: systems written in different languages can exchange the same text when they follow the same format and agree on what its fields mean.
JSON defines how data is written, not the complete meaning of a particular data model. For example, the format permits a field named score to hold a number, but it does not say whether that number is a test result, a rating, or something else. ECMA-404, Ecma International’s JSON standard, explicitly limits its goal to defining valid JSON syntax. The applications exchanging data must establish their own rules for required fields, allowed values, and interpretation.
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How JSON is structured
Objects: named values
An object is a collection of name/value pairs enclosed in curly braces, {}. Each member has a string name, followed by a colon and a value. Commas separate members. In the example, "active" is a name and true is its value.
Object member order is not semantically guaranteed by the JSON standard. Applications should identify members by their names rather than rely on their position in the text. A parser may preserve an order as an implementation detail, but that does not make order part of an object’s meaning under the standard.
Arrays: ordered values
An array is a sequence of values enclosed in square brackets, [], with commas between elements. Unlike object members, array elements are ordered: the first element is at index 0 in many programming languages, although the indexing convention belongs to the language using the parsed array. Array elements need not all have the same type.
["blue", 12, false, null]This array is valid JSON. Whether a particular application expects a mixed-type array is a separate question of application rules.
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The six JSON value types
- String: Text enclosed in double quotation marks, such as
"hello". - Number: A numeric value such as
12or-0.5, written using JSON’s number grammar. - Boolean: Either lowercase literal
trueorfalse. - Null: The lowercase literal
null, often used to represent an absent or empty value if the application assigns it that meaning. - Object: A collection of string-named values in curly braces.
- Array: An ordered sequence of values in square brackets.
JSON syntax rules that commonly cause errors
JSON is deliberately stricter than many programming-language notations and configuration formats. A parser accepting an extension does not make that extension portable JSON.
- Put double quotation marks around object names and string values. Single-quoted strings are not valid JSON.
- Separate each object name from its value with a colon, and separate members or array elements with commas.
- Use the lowercase literals
true,false, andnull. Capitalized or quoted versions are different text. - Do not add comments or a trailing comma after the last member or element. RFC 8259’s grammar does not permit either.
- Do not write leading zeros on ordinary numbers:
0is valid, but07is not.NaNandInfinityare not JSON numbers. - A JSON text can be any serialized JSON value, not just an object or array. Objects and arrays are simply common top-level forms.
For networked JSON exchange outside a closed ecosystem, RFC 8259 specifies UTF-8. Using standard syntax and UTF-8 improves interoperability across implementations.
Is JSON the same as JavaScript?
No. JSON’s design was derived from JavaScript object-literal conventions, and the names look familiar to JavaScript developers, but valid JSON is not any arbitrary JavaScript expression. JSON is data text: it does not include executable features such as assignments or function calls. JavaScript is a programming language with expressions and behavior beyond JSON’s limited data syntax.
For instance, the following is not JSON because it uses a single-quoted string and includes a function call:
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{ name: 'Mina', score: calculateScore() }The quoted object at the beginning of this article is JSON because its names and string values use double quotes, its values follow JSON’s grammar, and it contains no executable code.
Parsing JSON safely and checking what it means
Use a dedicated JSON parser to convert JSON text into values in your programming language. Do not use JavaScript eval() or a similar evaluation function to parse untrusted text. RFC 8259 warns that evaluation can execute code and is generally an unacceptable security risk. A JSON parser reads data according to JSON syntax rather than treating the input as a program.
A successful parse only shows that the text conforms to the parser’s accepted JSON syntax. It does not establish that the data is complete or appropriate for an application. A valid object might omit a field the application requires, give a field the wrong kind of value for that application, or contain an unrecognized value. Applications need their own validation and interpretation rules after parsing.
A practical validation sequence
- Parse first. Use the language’s JSON parser and handle its syntax errors. If parsing fails, inspect quotation marks, commas, colons, brackets, literal capitalization, and number spelling.
- Check the shape. Confirm that the top-level value is the expected kind, such as an object rather than an array.
- Validate application requirements. Check required member names, expected value types, allowed ranges, and any relationships between fields.
- Handle unexpected data deliberately. Decide how the application should respond to missing, extra, or invalid values rather than assuming every syntactically valid document is usable.
Using JSON with a web API
JSON commonly appears in API requests and responses. A request may encode settings as JSON, while a response may return information as JSON. The exact fields and required types come from that API’s documentation; JSON itself does not define them. For example, when an API accepts query parameters, those parameters are text values in a URL rather than a JSON document, even though the API may return a JSON response.
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Example: request a website screenshot
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Or skip the browser setup
One GET request can return a screenshot without setting up a browser locally:
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Common JSON problems and fixes
| Symptom | Likely cause | Fix |
|---|---|---|
| Parser flags a string or member name | Single quotes, missing double quotes, or an unescaped quotation mark | Use double quotes for JSON strings and names, and escape quotation marks inside a string as required by JSON syntax. |
| Parser reports an unexpected token near a comma | A trailing comma or missing comma between values | Remove a comma after the final item, or add the missing comma between adjacent items. |
true, false, or null is rejected |
The literal is capitalized or enclosed in quotes when a JSON value was intended | Use the exact lowercase literal without quotes. |
| A number is rejected | It uses a leading zero, NaN, or Infinity |
Write a number permitted by JSON’s grammar; represent any application-specific special case using an agreed JSON-compatible representation. |
| Parsing succeeds but the application rejects the document | Syntax is valid, but required fields or application rules are not satisfied | Validate the parsed value against the application’s expected shape and semantics. |
| Data behaves differently across systems | Implementations rely on nonstandard syntax, an assumed object-member order, or unspecified field meanings | Exchange standard JSON, use UTF-8 for networked exchange outside closed ecosystems, treat arrays as ordered and objects as named collections, and document field meanings separately. |
Standards behind JSON
The IETF’s RFC 8259 (STD 90), published in December 2017, describes JSON’s grammar, interoperability considerations, encoding guidance, media type, and security considerations. Ecma International’s ECMA-404, second edition, published in December 2017, defines JSON syntax and deliberately does not define application semantics or programming-language mappings. Together, these standards help distinguish what makes text JSON from what a specific system expects that JSON to mean.
Frequently Asked Questions
Does JSON have to start with curly braces?
No. A JSON text can represent any JSON value, including a string, number, boolean, null, object, or array.
Can an array contain different kinds of values?
Yes. JSON permits array elements of different types; whether an application accepts a particular mix is up to that application.
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Does valid JSON guarantee that a program can use the data?
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