问题

Can someone tell me what is the main difference between a JavaScript object defined by using Object Literal Notation and JSON object?

According to a JavaScript book it says this is an object defined by using Object Notation:

var anObject = {     property1 : true,     showMessage : function (msg) { alert(msg) } };

Why isn\'t it a JSON object in this case? Just because it is not defined by using quotation marks?

回答1:

Lets clarify first what JSON actually is. JSON is a textual, language-indepedent data-exchange format, much like XML, CSV or YAML.

Data can be stored in many ways, but if it should be stored in a text file and be readable by a computer, it needs to follow some structure. JSON is one of the many formats that define such a structure.

Such formats are typically language-independent, meaning they can be processed by Java, Python, JavaScript, PHP, you name it.

In contrast, JavaScript is a programming language. Of course JavaScript also provides a way to define/describe data, but the syntax is very specific to JavaScript.

As a counter example, Python has the concept of tuples, their syntax is (x, y). JavaScript doesn't have something like this.

Lets look at the syntactical differences between JSON and JavaScript object literals.

JSON has the following syntactical constraints:

Object keys must be strings (i.e. a character sequence enclosed in double quotes ").
The values can be either:
- a string
- a number
- an (JSON) object
- an array
- true
- false
- null
Duplicate keys ({"foo":"bar","foo":"baz"}) produce undefined, implementation-specific results; the JSON specification specifically does not define their semantics

In JavaScript, object literals can have

String literals, number literals or identifier names as keys (since ES6, keys can now also be computed, which introduces yet another syntax).
The values can be any valid JavaScript expression, including function definitions and undefined.
Duplicate keys produce defined, specified results (in loose mode, the latter definition replaces the former; in strict mode, it's an error).

Knowing that, just be looking at the syntax, your example is not JSON because of two reasons:

Your keys are not strings (literals). They are identifier names.
You cannot assign a function as a value to a "JSON object" (because JSON doesn't define any syntax for functions).

But most importantly, to repeat my explanation from the beginning: You are in a JavaScript context. You define a JavaScript object. If any, a "JSON object" can only be contained in a string:

 var obj = {foo: 42}; // creates a JavaScript object (this is *not* JSON)  var json = '{"foo": 452}'; // creates a string containing JSON

That is, if you're writing JavaScript source code, and not dealing with a string, you're not dealing with JSON. Maybe you received the data as JSON (e.g., via ajax or reading from a file), but once you or a library you're using has parsed it, it's not JSON anymore.

Only because object literals and JSON look similar, it does not mean that you can name them interchangeably. See also There's no such thing as a "JSON Object".

回答2:

JSON has a much more limited syntax including:

Key values must be quoted
Strings must be quoted with " and not '
You have a more limited range of values (e.g. no functions allowed)

回答3:

There is really no such thing as a "JSON Object".

The JSON spec is a syntax for encoding data as a string. What people call a "JSON Object" ( in javascript ) is really just an ordinary javascript object that has (probably) been de-serialized from a valid JSON string, and can be easily re-serialized as a valid JSON string. This generally means that it contains only data ( and not functions ). It also means that there are no dates, because JSON does not have a date type ( probably the most painful thing about JSON ;)

Furthermore, (side-rant...) when people talk about a "JSON Object", they almost always mean data that has the "curly-braces" at the top-level. This corresponds nicely to a javascript object. However, the JSON spec does not require that there be a single "curly-braces" object at the top-level of a JSON string. It is perfectly valid JSON to have a list at the top-level, or even to have just a single value. So, while every "JSON Object" corresponds to valid JSON, not all valid JSON strings correspond to what we would call a "JSON Object"! ( because the string could represent a list or an atomic value )

回答4:

According to JSON in JavaScript,

JSON is a subset of the object literal notation of JavaScript.

In other words, valid JSON is also valid JavaScript object literal notation but not necessarily the other way around.

In addition to reading the documentation, as @Filix King suggested, I also suggest playing around with the JSONLint online JSON validator. That's how I learned that the keys of JSON objects must be strings.

回答5:

🔫 JSON: The Fat-Free Alternative to XML

JSON has been widely adopted by people who found that it made it a lot easier to produce distributed applications and services. The official Internet media type for JSON is application/json RFC 4627. JSON filenames use the extension .json.

► JavaScript Object Notation (JSON) is a lightweight, text-based, language-independent data interchange format. JSON has been used to exchange data between applications written in any Programming language.

The JSON object is a single object that contains two functions, parse and stringify, that are used to parse and construct JSON texts.

JSON.stringify produces a String that conforms to the following JSON grammar.

JSON.parse accepts a String that conforms to the JSON grammar.

The parseJSON method will be included in the Fourth Edition of ECMAScript. In the meantime, a JavaScript implementation is available at json.org.

var objLiteral = {foo: 42}; // JavaScript Object console.log('Object Literal : ', objLiteral ); // Object {foo: 42}foo: 42__proto__: Object  // This is a JSON String, like what you'd get back from an AJAX request. var jsonString = '{"foo": 452}'; console.log('JOSN String : ', jsonString ); // {"foo": 452}  // This is how you deserialize that JSON String into an Object. var serverResposnceObject = JSON.parse( jsonString ); console.log('Converting Ajax response to JavaScript Object : ', serverResposnceObject); // Object {foo: 42}foo: 42 __proto__: Object  // And this is how you serialize an Object into a JSON String. var serverRequestJSON = JSON.stringify( objLiteral ); console.log('Reqesting server with JSON Data : ', serverRequestJSON); // '{"foo": 452}'

JSON is subset of JavaScript. Javascript was derived from the ECMAScript Programming Language Standard.

► ECMAScript

ECMAScript has grown to be one of the world's most widely used general purpose programming languages. It is best known as the language embedded in web browsers but has also been widely adopted for server and embedded applications. ECMAScript is based on several originating technologies, the most well-known being JavaScript (Netscape Communications)) and JScript (Microsoft Corporation).). Though before 1994, ECMA was known as "European Computer Manufacturers Association", after 1994, when the organization became global, the "trademark" "Ecma" was kept for historical reasons.

ECMAScript is the language, whereas JavaScript, JScript, and even ActionScript are called "Dialects".

Dialects have been derived from the same language. They are are quite similar to each other as they have been derived from the same language but they have undergone some changes. A dialect is a variation in the language itself. It is derived from a single language.

SQL Language - Hibernate MySQL Dialect, Oracle Dialect,.. which have some changes or added functionality.

Information about the browser and computer of your users.

navigator.appName // "Netscape"

ECMAScript is the scripting language that forms the basis of JavaScript. JavaScript language resources.

ECMA-262 Links
Initial Edition, June 1997 PDF.
2nd Edition, August 1998 PDF.
3rd Edition, December 1999 PDF.
5th Edition, December 2009 PDF.
5.1 Edition, June 2011 HTML.
6th Edition, June 2015 HTML.
7ᵗʰ Edition, June 2016 HTML.
8th edition, June 2017 HTML.
9th Edition, 2018 HTML.

NOTE « 4th edition of ECMAScript not published as the work was incomplete.

JSON defines a small set of formatting rules for the portable representation of structured data.

► Key values must be quoted, only Strings are allowed for keys. If you use other than String it will convert to String. But not recommended to use keys other than String's. Check an example like this - { 'key':'val' } over RFC 4627 - jsonformatter

var storage = {   0 : null,   1 : "Hello" }; console.log( storage[1] ); // Hello console.log( JSON.stringify( storage ) ); // {"0":null,"1":"Hello","2":"world!"}  var objLiteral = {'key1':'val1'};     var arr = [10, 20], arr2 = [ 'Yash', 'Sam' ];     var obj = { k: 'v' }, obj2 = { k2: 'v2' };     var fun = function keyFun() {} ;  objLiteral[ arr ] = 'ArrayVal';     objLiteral[ arr2 ] = 'OverridenArrayVal'; objLiteral[ obj ] = 'ObjectVal';    objLiteral[ obj2 ] = 'OverridenObjectVal'; objLiteral[ fun ] = 'FunctionVal';  console.log( objLiteral ); // Object {key1: "val1", 10,20: "ArrayVal", Yash,Sam: "OverridenArrayVal", [object Object]: "OverridenObjectVal", function keyFun() {}: "FunctionVal"} console.log( JSON.stringify( objLiteral ) ); // {"key1":"val1","10,20":"ArrayVal","Yash,Sam":"OverridenArrayVal","[object Object]":"OverridenObjectVal","function keyFun() {}":"FunctionVal"} console.log( JSON.parse( JSON.stringify( objLiteral ) ) ); // Object {key1: "val1", 10,20: "ArrayVal", Yash,Sam: "OverridenArrayVal", [object Object]: "OverridenObjectVal", function keyFun() {}: "FunctionVal"}  console.log('Accessing Array  Val : ', objLiteral[ [10,20] ] ); console.log('Accessing Object Val : ', objLiteral[ '[object Object]' ] ); console.log('Accessing Function Val : ', objLiteral[ 'function keyFun() {}' ] );

► JSON Strings must be quoted with " and not '. A string is very much like a C or Java string. Strings should be wrapped in double quotes.
- Literals are fixed values, not variables, that you literally provide in your script.
- A string is a sequence of zero or more characters wrapped in quotes with backslash escapement, the same notation used in most programming languages.
  - 🔫 - Special Symbols are allowed in String but not recomended to use.
  - \" - Special characters can be escaped. But not recomended to escape (') Single Quotes. In Strict mode it will throw and Error - SyntaxError: Unexpected token ' in JSON
Check with this code { "Hai\" \n Team 🔫":5, "Bye \'": 7 } over online JSON Edtions. ModesnotStrict,Strinct.
```
var jsonString = "{'foo': 452}"; // {'foo': 452} var jsonStr = '{"foo": 452}'; // {"foo": 452}  JSON.parse( jsonString ); // Unexpected token ' in JSON at position 1(…) JSON.parse( jsonStr ); // Object {foo: 452}  objLiteral['key'] = 'val'; // Object {foo: 42, key: "val"} objLiteral.key2 = 'val';  // objLiteral.key\n3 - SyntaxError: Invalid or unexpected token objLiteral['key\n3'] = 'val'; // Object {"foo": "42", key: "val", key2: "val", "key↵3": "val"}  JSON.stringify( objLiteral ); // {"foo":"42","key":"val","key2":"val","key\n3":"val"} 
```

Object Property accessors provide access to an object's properties by using the dot notation or the bracket notation.

► You have a more limited range of values (e.g. no functions allowed). A value can be a string in double quotes, number, boolean, null, object, or array. These structures can be nested.

var objLiteral = {}; objLiteral.funKey = function sayHello() {     console.log('Object Key with function as value - Its outcome message.'); };  objLiteral['Key'] = 'Val';  console.log('Object Literal Fun : ', objLiteral ); // Object Literal Fun :  Object {Key: "Val"}Key: "Val"funKey: sayHello()__proto__: Object console.log( JSON.stringify( objLiteral ) ); // {"Key":"Val"}

► JavaScript is the most popular implementation of the ECMAScript Standard. The core features of Javascript are based on the ECMAScript standard, but Javascript also has other additional features that are not in the ECMA specifications/standard. Every browser has a JavaScript interpreter.

JavaScript is a dynamically typed language. That means you don't have to specify the data type of a variable when you declare it, and data types are converted automatically as needed during script execution.

Literals :

'37' - 7    // 30 '37' + 7    // "377" +'37' + 7   // 44 +'37'       // 37 '37'        // "37"  parseInt('37');     // 37 parseInt('3.7');    // 3  parseFloat(3.7);    // 3.7  // An alternative method of retrieving a number from a string is with the + (unary plus) operator: +'3.7'              // 3.7

Object literals RFC 7159

An object structure is represented as a pair of curly brackets surrounding zero or more name/value pairs (or members). A name is a string. A single colon comes after each name, separating the name from the value. A single comma separates a value from a following name. The names within an object SHOULD be unique.

ECMAScript supports prototype-based inheritance. Every constructor has an associated prototype, and every object created by that constructor has an implicit reference to the prototype (called the object’s prototype) associated with its constructor. Furthermore, a prototype may have a non-null implicit reference to its prototype, and so on; this is called the prototype chain.

In a class-based object-oriented language, in general, state is carried by instances, methods are carried by classes, and inheritance is only of structure and behavior. In ECMAScript, the state and methods are carried by objects, and structure, behavior, and state are all inherited.

A prototype is an object used to implement structure, state, and behavior inheritance in ECMAScript. When a constructor creates an object, that object implicitly references the constructor’s associated prototype for the purpose of resolving property references. The constructor’s associated prototype can be referenced by the program expression constructor.prototype, and properties added to an object’s prototype are shared, through inheritance, by all objects sharing the prototype.

回答6:

For the ones who still think that RFC are more important than blogs and opinion based misconceptions, let's try to answer clarifying some points. I'm not going to repeat all the correct differences already mentioned in previous answers, here I'm just trying adding value summarizing some crucial part rfc7159

Extracts from https://tools.ietf.org/html/rfc7159

JavaScript Object Notation (JSON) is a text format for the serialization of structured data. It is derived from the object literals of JavaScript, as defined in the ECMAScript Programming Language Standard, Third Edition [ECMA-262].
JSON can represent four primitive types (strings, numbers, booleans, and null) and two structured types (objects and arrays).
An object is an unordered collection of zero or more name/value pairs, where a name is a string and a value is a string, number, boolean, null, object, or array.
begin-object = ws %x7B ws ; { left curly bracket
end-object = ws %x7D ws ; } right curly bracket
A JSON value MUST be an object, array, number, or string, or one of the following three literal names: false null true
An object structure is represented as a pair of curly brackets
The names within an object SHOULD be unique. object = begin-object [ member *( value-separator member ) ] end-object
An object whose names are all unique is interoperable in the sense that all software implementations receiving that object will agree on the name-value mappings. When the names within an object are not unique, the behavior of software that receives such an object is unpredictable.

Examples (from page 12 of RFC)

This is a JSON object:

      {         "Image": {             "Width":  800,             "Height": 600,             "Title":  "View from 15th Floor",             "Thumbnail": {                 "Url":    "http://www.example.com/image/481989943",                 "Height": 125,                 "Width":  100             },             "Animated" : false,             "IDs": [116, 943, 234, 38793]           }       }

Its Image member is an object whose Thumbnail member is an object and whose IDs member is an array of numbers.

There is really no such thing as a "JSON Object".

Really?

回答7:

As far as I understand the main difference is the flexibility.

JSON is a kind of wrapper on "JavaScript Object Notation" which forces users to obey more strict rules for defining the objects. And it does this by limiting the possible object declaration ways provided by JavaScript Object Notation feature.

As a result we have a simpler and more standardized objects which suits better on data-exchange between platforms.

So basically, the newObject in my example above is an object defined by using JavaScript Objeect Notation; but it is not a 'valid' JSON object because it does not follow the rules that JSON standards require.

This link is also quite helpful: http://msdn.microsoft.com/en-us/library/bb299886.aspx

回答8:

First you should know what JSON is:

It is language agnostic data-interchange format. The syntax of JSON was inspired by the JavaScript Object Literal notation, but there are differences between them.

For example, in JSON all keys must be quoted, while in object literals this is not necessary:

// JSON: { "foo": "bar" }

// Object literal: var o = { foo: "bar" }; The quotes are mandatory on JSON because in JavaScript (more exactly in ECMAScript 3rd. Edition), the usage of reserved words as property names is disallowed, for example:

var o = { if: "foo" }; // SyntaxError in ES3 While, using a string literal as a property name (quoting the property name) gives no problems:

var o = { "if": "foo" }; So for "compatibility" (and easy eval'ing maybe?) the quotes are mandatory.

The data types in JSON are also restricted to the following values:

string number object array A literal as: true false null The grammar of Strings changes. They have to be delimited by double quotes, while in JavaScript, you can use single or double quotes interchangeably.

// Invalid JSON: { "foo": 'bar' } The accepted JSON grammar of Numbers also changes, in JavaScript you can use Hexadecimal Literals, for example 0xFF, or (the infamous) Octal Literals e.g. 010. In JSON you can use only Decimal Literals.

// Invalid JSON: { "foo": 0xFF }

回答9:

Javascript Object Literal vs JSON:

Object literal syntax is a very convenient way to create javascript objects
The JSON language, which stands for 'Javascript object notation', has its syntax derived from javascript object literal syntax. It is used as a programming language independent textual data transfer format.

Example:

JS object notation, used in JS to create objects in the code conveniently:

const JS_Object = {   1: 2,  // the key here is the number 1, the value is the number 2   a: 'b', // the key is the string a, the value is the string b   func: function () { console.log('hi') }   // the key is func, the value is the function }

Example of JSON:

{"widget": {     "debug": "on",     "window": {         "title": "Sample Konfabulator Widget",         "name": "main_window",         "width": 500,         "height": 500     },     "image": {          "src": "Images/Sun.png",         "name": "sun1",         "hOffset": 250,         "vOffset": 250,         "alignment": "center"     },     "text": {         "data": "Click Here",         "size": 36,         "style": "bold",         "name": "text1",         "hOffset": 250,         "vOffset": 100,         "alignment": "center",         "onMouseUp": "sun1.opacity = (sun1.opacity / 100) * 90;"     } }}

Main differences:

All object keys in JSON must be strings. In Javascript object keys can be strings or numbers
All strings in JSON must be quoted in "double quotes". Whereas in Javascript both single quotes and double quotes are allowed. Even with no quotes in the Javascript object notation the object keys are implicitly casted to strings.
In JSON a function cannot be defined as a value of an object (since this is Javascript specific). In Javascript this is completely legal.

Javascript build in JSON object:

JSON objects can be easily converted to Javascript and vice versa using the built in JSON object which Javascript offers in its runtime. For example:

const Object = {    property1: true,    property2: false,  }; // creating object with JS object literal syntax    const JSON_object = JSON.stringify(Object);  // stringify JS object to a JSON string    console.log(JSON_object); // note that the (string) keys are in double quotes    const JS_object = JSON.parse(JSON_object);  // parse JSON string to JS object    console.log(JS_object.property1, JS_object.property2);   // accessing keys of the newly created object

回答10:

Here is one surprising difference: you can not use undefined in json and all object fields with undefined values will disappear after JSON.stringify

let object =  { "a": undefined } ;    let badJSON= '{ "a": undefined }';      console.log('valid JS object :', object );  console.log('JSON from object:', JSON.stringify(object) );  console.log('invalid json    :', JSON.parse(badJSON) );

🙈🙉🙊

来源：https://stackoverflow.com/questions/2904131/what-is-the-difference-between-json-and-object-literal-notation

标签

javascript

json

object-notation

What is the difference between JSON and Object Literal Notation?

问题