What is the difference between JSON and Object Literal Notation?

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.

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 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.

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”.

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