JSON and Data Validation

JSON is a text format for objects, arrays, strings, numbers, booleans, and null. It does not preserve functions, undefined, symbols, Map, Set, or class identity.

Parse and stringify

01const payload = JSON.stringify({ course: "JavaScript", complete: false });
02
03let value;
04try {
05  value = JSON.parse(payload);
06} catch {
07  throw new Error("Response was not valid JSON");
08}

Parsing confirms syntax, not meaning. External data can still have missing properties or incorrect types.

Validate the boundary

01function isCourse(value) {
02  return typeof value === "object" && value !== null
03    && typeof value.title === "string"
04    && Number.isInteger(value.lessonCount)
05    && value.lessonCount >= 0;
06}
07
08if (!isCourse(value)) throw new TypeError("Invalid course response");

For larger systems, use a schema validator and derive static types from the same schema when possible.

Keep this

Know what JSON can represent, handle syntax errors, and validate shape before data enters the rest of the application.