Question

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?

Answer 1

Lets clarify first what JSON actually is. JSON is a textual, language-independent 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.

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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).

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

1. Your keys are not strings (literals). They are identifier names.
2. 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.

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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”.