What Defines Valid JSON: RFC 4627 vs. ECMA-404?

What Constitutes the Bare Minimum of Valid JSON?

The definition of JSON's syntax has been subject to subtle variations over time, leading to confusion regarding the precise criteria for valid JSON strings.

RFC 4627 and Early Implementations

Initially, JSON's specification, as defined in RFC 4627, strictly limited valid JSON to serialized objects ({}) and arrays([]). This meant that simple values like strings, numbers, true, and false were not considered valid JSON on their own.

ECMA-404 and Browser Implementations

However, the introduction of ECMA-404 expanded the definition of JSON to include top-level JSONValues, which encompass all JSON data types. As a result, web browsers now allow all JSON values as valid JSON.

Browser Exceptions vs. RFC Compliance

It's important to note that browsers implement the JSON object based on ES5, which allows scalar types and NULL as valid JSON values. This is an exception from the original RFC 4627 specification. Consequently, some other implementations, such as Ruby, adhere strictly to RFC 4627, resulting in different interpretations of valid JSON.

Practical Implications

Understanding these nuances is crucial, especially when working with different JSON parsers and encoders. It's essential to confirm whether the implementation strictly follows RFC 4627 or adopts the more permissive ECMA-404 definition, as this affects the validation of JSON strings.

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