One-call validation with ajv.validate

ajv.validate checks data against a schema in one call, without a separate compile step, and leaves the errors on the Ajv instance.

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One-call validation with ajv.validate: screenshot of the example below running — The code below, running in Chromium on our server (Oct 12, 2026). Press Run to see it live and change it.

How it works

Validate in one call. ajv.validate(productSchema, p) takes the schema and the data together and returns a boolean. The schema is compiled and cached on the first call. [1]
Read errors from the instance. With this method the errors live on ajv.errors rather than on a function. For the valid product the example prints null. [1]
Copy errors you need later. Every call replaces ajv.errors, so the loop pushes each error array into saved and prints them all at the end. [1]

Run and edit it

It runs in your browser, in a sandboxed frame that cannot see this site; the packages load from esm.sh only after you press Run.

import Ajv from "ajv"

const ajv = new Ajv()

const productSchema = {
  type: "object",
  properties: {
    sku: { type: "string", pattern: "^[A-Z]{3}-[0-9]{3}$" },
    price: { type: "number", exclusiveMinimum: 0 },
    tags: { type: "array", items: { type: "string" }, maxItems: 3 },
  },
  required: ["sku", "price"],
}

const products = [
  { sku: "ABC-123", price: 9.5, tags: ["new"] },
  { sku: "abc-1", price: 4 },
  { sku: "XYZ-999", price: 0 },
  { sku: "XYZ-999", price: 5, tags: ["a", "b", "c", "d"] },
  { price: 12 },
]

const saved = []
products.forEach((p, i) => {
  // no compile step: the schema is passed with the data
  const ok = ajv.validate(productSchema, p)
  console.log(`product ${i + 1}: ${ok ? "valid" : "invalid"}`)
  if (ok) console.log("  ajv.errors =", ajv.errors)
  else saved.push([i + 1, ajv.errors]) // keep a reference before the next call
})

console.log("--- saved errors ---")
for (const [n, errs] of saved) {
  console.log(`product ${n}:`, ajv.errorsText(errs))
}

Ctrl/⌘ + Enter runs

Remove the saved.push line and print ajv.errors after the loop to see that only the last result is left. 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

Compiling a schema with Ajv Collecting all errors with Ajv Reusing schemas with addSchema and $ref Custom formats in Ajv

Sources

Docs: api.html

Explanations written by AgentGid from the official docs; the numbers in brackets point here. The code marked "written by AgentGid" is ours and free to copy.

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