JSON and data

In plain English

JSON (JavaScript Object Notation) is a text format for storing and sharing data, built from objects, arrays, strings, numbers, booleans and null. JSON.stringify turns a JavaScript value into a JSON string, and JSON.parse turns a JSON string back into a value. A JSON value can nest objects inside arrays and arrays inside objects to represent rich data. When a string is not valid JSON, JSON.parse throws an error, so you wrap it in a try/catch block to fail gracefully instead of crashing.

Step by step

1. 1Create a JavaScript object with a nested object and an array.
2. 2Turn it into a string with JSON.stringify.
3. 3Turn the string back into an object with JSON.parse.
4. 4Wrap a JSON.parse call in try/catch and confirm a bad string is caught.

const profile = {
  name: "Amina",
  age: 28,
  hobbies: ["reading", "cycling"],
  address: { city: "Cairo", country: "Egypt" }
};

const text = JSON.stringify(profile);
console.log(text);

const restored = JSON.parse(text);
console.log(restored.address.city);

try {
  JSON.parse("this is not json");
} catch (err) {
  console.log("Invalid JSON caught");
}

Common mistakes

• Forgetting that JSON keys must be double-quoted strings, unlike JavaScript object literals.
• Calling JSON.parse on a string that is not JSON and getting an uncaught error.
• Confusing a JSON string with the object it represents and forgetting to parse it first.
• Trying to put functions or comments inside JSON, which the format does not allow.