Validator

A validator is a tool that checks a document against a set of rules and reports any violations. There are two common layers:

1. Syntactic validation — does the document parse at all? For JSON, this means every brace is matched, every string is properly quoted, every number is well-formed, and so on. This is the minimum bar.
2. Schema validation — does the document match an expected shape? For JSON, this typically means validating against JSON Schema. For XML, it means validating against an XSD, DTD, or RELAX NG grammar.

A syntactic validator will reject {"a": 1,} because of the trailing comma. A schema validator will reject {"name": 42} if the schema says name must be a string — syntactically the document is fine, but semantically it is wrong.

Example workflow

import Ajv from "ajv";
const ajv = new Ajv();
const schema = { type: "object", required: ["id"], properties: { id: { type: "integer" } } };
const validate = ajv.compile(schema);

const candidate = { id: "forty-two" };
if (!validate(candidate)) {
  console.error(validate.errors);
  // [{ instancePath: "/id", schemaPath: "#/properties/id/type",
  //    keyword: "type", params: { type: "integer" }, message: "must be integer" }]
}

The output tells you exactly which field failed, why, and where in the schema the rule lives.

Where validators fit in the pipeline

• Input boundaries — incoming HTTP requests, webhook payloads, messages from a queue. Validating at the boundary catches bad data before it reaches business logic.
• Output boundaries — outgoing responses. Many teams validate outgoing data in development only, to catch bugs in serialisation code.
• Build time — validating config files, manifests, and other declarative inputs as part of CI.
• Editor time — $schema-aware editors validate in real time as you type.

The difference between “parses” and “valid”

A common confusion: “my JSON parses in JavaScript, so it is valid”. Not quite. A parser turns bytes into an in-memory value; a validator checks the value against rules. JSON.parse("{}") succeeds, but your API may still reject {} because it required certain fields. Always run both.

Tools

• Syntax validators: the JSON tool here pressing the “Validate” button, jsonlint, jq -e ..
• Schema validators: Ajv (JavaScript), jsonschema (Python), json-schema-rs (Rust), santhosh-tekuri/jsonschema (Go).
• Online schema validators: jsonschemavalidator.net, but prefer local libraries once you are in production.