JSON In, JSON Out

The data format of the web

APIs exchange JSON (JavaScript Object Notation): a text format for objects, arrays, strings, numbers, booleans, and null. It looks like JavaScript literals with two extra rules: keys are always double-quoted, and no trailing commas, comments, or functions.

Networks only carry bytes of text, so objects must be converted both ways:

• JSON.stringify(obj) turns a live object into a JSON string (to send).
• JSON.parse(text) turns a JSON string back into an object (to receive).

Express's res.json(obj) calls stringify for you, and express.json() middleware calls parse on incoming bodies. Underneath, it is these two functions.

Parse can explode

JSON.parse throws an exception on malformed input, and clients send malformed JSON all the time (truncated requests, hand-written curl commands, buggy apps). An unhandled throw in a server means a crashed request, so parsing untrusted text always gets a try/catch:

try {
  const body = JSON.parse(text);
} catch (err) {
  // respond 400 invalid_input, using the error shape from lesson 4-3
}

A tidy pattern wraps this in a helper that never throws and returns a result object instead. Build it below, this shape ({ ok, value } or { ok, error }) appears all over production Node code.