What Is JSON? The Data Format of Modern APIs

What You'll Learn

• What JSON is and why it is everywhere
• The six data types JSON supports
• How to read and write JSON
• Why JSON beat XML for APIs

Why This Matters

Almost every modern API sends and receives data as JSON. If you cannot read JSON, you cannot read API responses. If you cannot write JSON, you cannot send API requests. This lesson gives you the format in one sitting.

Simple Explanation

JSON (JavaScript Object Notation) is a way to write data as plain text so that both humans and computers can read it. It looks like this:

{
  "name": "Anita",
  "age": 28,
  "is_active": true,
  "tags": ["vip", "early-adopter"],
  "address": null
}

That is JSON. It is just text — but it has a strict structure that every programming language can parse.

Real-World Analogy — A Filled-Out Form

Think of a job application form. Each field has a label ("Name", "Age", "Email") and a value you fill in. JSON works the same way: each piece of data has a label (called a key) and a value.

The form is structured — you don't write your age in the "Name" field. JSON enforces the same structure: keys and values are paired, and the whole thing is wrapped in curly braces.

The Six JSON Data Types

Rules You Must Follow

1. Keys must be in double quotes. {"name": "Anita"} is valid; {name: "Anita"} is not (in strict JSON).
2. Strings must be in double quotes. Single quotes are not allowed: 'Anita' is invalid.
3. No trailing commas. {"a": 1, "b": 2,} is invalid — remove the last comma.
4. No comments. // this is a comment is not allowed in JSON.
5. Boolean and null are lowercase. True, TRUE, Null, NULL are all invalid.

Reading JSON — Step by Step

{
  "id": 123,
  "name": "Anita",
  "email": "anita@example.com",
  "is_active": true,
  "roles": ["admin", "editor"],
  "address": {
    "city": "Mumbai",
    "country": "India"
  },
  "phone": null
}

Read it as:

• id is a number: 123
• name is a string: "Anita"
• email is a string: "anita@example.com"
• is_active is a boolean: true
• roles is an array of two strings
• address is an object with two keys (city, country)
• phone is null (no value provided)

Why JSON Beat XML

Before JSON, the standard was XML. XML works but is verbose:

<user>
  <id>123</id>
  <name>Anita</name>
  <email>anita@example.com</email>
</user>

The same data in JSON:

{"id": 123, "name": "Anita", "email": "anita@example.com"}

JSON is smaller, faster to parse, and easier to read. XML is still used in some enterprise and SOAP APIs (covered in Module 12), but JSON is the default for new APIs.

Common Mistakes

1. Using single quotes. {'name': 'Anita'} works in JavaScript but is invalid JSON.
2. Trailing commas. Most parsers reject them. JSON validators flag them.
3. Comments in JSON. If you need comments, use a separate documentation file or JSONC (a non-standard extension).
4. Unquoted keys. {name: "Anita"} is JavaScript, not JSON.
5. Using NaN or Infinity. JSON has no representation for these. Use null instead.

Practical Exercise (5 minutes)

Copy this JSON into a text file and try to identify all the data types:

{
  "product": "Laptop",
  "price": 95000,
  "in_stock": true,
  "tags": ["electronics", "computers"],
  "discount": null,
  "specs": {"ram": 16, "ssd": 512}
}

Answers: product=string, price=number, in_stock=boolean, tags=array, discount=null, specs=object.

Mini Challenge

Write valid JSON for a "book" with these fields: title, author, year, price, in_print (boolean), genres (array of strings), and publisher (object with name and country). Validate your JSON at jsonlint.com.

Key Takeaways