The 5 Mistakes That Break JSON (and How to Fix Them Fast)

Quick answer: almost every "invalid JSON" error comes from five habits carried over from JavaScript. JSON is stricter than JS: keys and strings need double quotes, there is no trailing comma, and there are no comments.

The five mistakes

Find the exact error in code

JSON.parse tells you roughly where it failed:

function checkJson(text) {
  try {
    return { ok: true, value: JSON.parse(text) };
  } catch (e) {
    return { ok: false, error: e.message }; // e.g. "Unexpected token } in JSON at position 17"
  }
}

checkJson('{"a": 1,}'); // { ok: false, error: "..." }

The position in the message is a character offset. Count to it, or paste the text into a formatter, to see the line.

Pretty print or minify

const pretty = JSON.stringify(value, null, 2); // readable, 2 spaces
const small  = JSON.stringify(value);          // minified, one line

Pretty printing is for people. Minified JSON is smaller to send.

Two gotchas

• undefined, functions and NaN are not valid JSON values. JSON.stringify drops undefined and functions from objects and turns NaN into null.
• Big numbers lose precision in JavaScript because they are 64-bit floats. If an ID has more than 15 digits, send it as a string.

A free tool for it

If you just need to paste and check, I keep a free browser-based JSON formatter and validator. It formats, minifies and shows the error, and nothing is uploaded. I also wrote a plain guide with the same table: How to format JSON online and fix common errors.

Originally published at saveanyreel.com.