Compiling a schema with Ajv

How it works

1. Describe the data with a schema. The schema requires an integer foo, allows an optional string bar, and rejects any other property through additionalProperties: false. [1]
2. Compile once, validate many times. ajv.compile(schema) returns a function that this example reuses for all six samples. The docs advise compiling each schema once and reusing the function. [1]
3. Read the errors. The function returns true or false. When it fails, validate.errors holds the error objects, and ajv.errorsText() turns them into one readable line. [2]

Run and edit it

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import Ajv from "ajv"

const ajv = new Ajv()

const schema = {
  type: "object",
  properties: {
    foo: { type: "integer" },
    bar: { type: "string" },
  },
  required: ["foo"],
  additionalProperties: false,
}

// compile once, then reuse the function for every object
const validate = ajv.compile(schema)

const samples = [
  { foo: 1, bar: "abc" },
  { foo: 42 },
  { bar: "abc" },
  { foo: 1.5 },
  { foo: 2, bar: 7 },
  { foo: 3, extra: true },
]

for (const data of samples) {
  const valid = validate(data)
  console.log(JSON.stringify(data).padEnd(22), valid ? "valid" : "invalid")
  if (!valid) console.log("   ", ajv.errorsText(validate.errors))
}

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Add a sample such as { foo: "7" } or make bar required and watch which lines turn invalid. Code written by AgentGid [1].

Checked Oct 12, 2026: it starts without errors in Chromium with ajv 8.20.0. Needs WebGL.

More Ajv examples

Collecting all errors with Ajv One-call validation with ajv.validate Reusing schemas with addSchema and $ref Custom formats in Ajv