JSON

In short: JSON (JavaScript Object Notation) is the text format almost every language and web service uses for structured data. JSON.stringify turns a live JavaScript value into JSON text, and JSON.parse turns that text back into a value. JSON is stricter than JavaScript object syntax: double quotes only, no functions, no undefined, no trailing commas. Anything JSON cannot represent — a function, undefined — is silently dropped by stringify, and parse always builds a brand-new value, never the original one back.

Save a playlist as text and load it back

Nothing outside JavaScript can hold a live object; it can only hold text. stringify and parse are the two functions that cross that boundary:

text is a string — you could save it to localStorage, send it over the network, or write it to a file. back is a new object built fresh from that string, and it behaves like any other object: back.count + 1 is 4.

Stringify a function and a missing value

JSON has no concept of a function or of undefined. stringify does not error on them — it simply leaves them out:

This prints {"a":1} — the b property vanishes entirely, rather than becoming null or the text "undefined". The same happens to any method on an object you stringify: functions never survive the trip through JSON.

Parse a value and lose its original type

JSON.parse returns exactly the type that was written in the text — including when that type is not the one you expected:

This prints "51", not 6. The JSON text stored "5" in quotes, so it is a string both before and after parsing, and + with a string joins text instead of adding numbers. JSON keeps exactly the types it was given — if a value needs to be a number, it must be written as one, with no quotes: {"plays": 5}.

Quick practice

🎯 Your turn

You save { name: "Evening Mix", count: 3, shuffle: undefined, play() {} } with JSON.stringify and load it back with JSON.parse. Explain what comes back and why.

Write your explanation and submit it — there's no code to run here, just your reasoning.