JSON Parser — Concepts & Architecture

Goal:

Build a standards-first JSON parser in TypeScript with:

• DOM
• streaming APIs
• strict-by-default behavior

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Introduction:

JSON (JavaScript Object Notation) is a lightweight data-interchange format used for exchanging or transfer data over the Internet.

Question 1. What JSON Document looks like ?

Top Level

• A JSON document is 1 value. It can be either any or all:

  • object {...}
  • array [...]
  • string "text"
  • number 42, -3.14, 1e6
  • boolean true / false
  • null null

Note: Most APIs use an object or array at the top level, but any single value is allowed.

Question 2. What JSON Objects looks like ?

• Curly braces with comma-separated key–value pairs: { "key": value, "another": value }

• Rules:

  • Keys must be double-quoted strings.
  • : -> separates key and value.
  • Order is not guaranteed.
  • Duplicate keys are discouraged (parsers often keep the last one).

Example:

{
    "id" : "A1234",
    "name" : "KUMAR",
    "isActive" : true,
    "contact" : 74567890,
    "tags": ["eng", "ai"],
    "profile": { "email": "nkunal@aol.com", "country": "India" }
}

Question 3. What JSON Arrays looks like ?

• Square brackets with comma-separated values: [ value1, value2, value3 ]

• Values can be any JSON type (including nested arrays/objects).

Example

[
    {"id" : 123, "name" : "KK"},
    {"id" : 898, "name" : "KumarK"}
]

Question 4. What counts as a JSON value ?

Exactly one of: object, array, string, number, true, false, null.

Question 5. Where is whitespace allowed and how is it treated ?

Spaces, tabs, and newlines are allowed between tokens and are ignored by parsers.

Inside strings, whitespace is part of the value

Question 6. What values, keys or types are not allowed in JSON ?

Not allowed in standard JSON:

• Comments (// or /* */)
• Trailing commas ([1,2,], {"a":1,})
• Single quotes for strings or keys
• Unquoted keys ({ name: "Ada" })
• Special numbers (NaN, Infinity)
• Leading zeros in numbers (012)

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Parsing

• The overall process of turning input text into a structured form your program can use.

• In a JSON parser, parsing means: text → (tokens →) JS values/events.

Mini-picture

"{"id":1}"  --parse-->  { id: 1 }