Thanks for the great library! I've recently started migrating from tv4 to ajv, primarily because of the promised speed improvements. Thus far AJV appears to be very feature complete and I like pre-registering all schemas in a single instance beforehand; keeps my schemas managed centrally in the codebase which is nice.

I noticed one thing that might be a cause of tricky and hard to reproduce bugs when using the library improperly.

When accessing ajv.errors (or ajv.errorsText I presume as well) in a callback, promise or any tick after the one in which you've called ajv.validate there is no guarantee that ajv.errors contains the set of errors you might expect.

Example:

var async = require('async');
var Ajv = require('ajv');

var ajv = new Ajv();
ajv.addSchema({'type': 'string'}, 'testSchema');

function test(name, timeout, data, done) {
  var valid = ajv.validate('testSchema', data);
  console.log(name, 'valid =', valid + ', ajv.errors =', ajv.errors);
  setTimeout(function() {
    console.log(name + ' timeout', ' valid =', valid + ', ajv.errors =', ajv.errors);
    done();
  } , timeout);
}

async.parallel([
  function(cb) {
    test('Test 1', 100, false, cb);
  },
  function(cb) {
    test('Test 2', 50, 'test', cb);
  },
]);

This outputs:

Test 1 valid = false, ajv.errors = [ { keyword: 'type', dataPath: '', message: 'should be string' } ]
Test 2 valid = true, ajv.errors = null
Test 2 timeout  valid = true, ajv.errors = null
Test 1 timeout  valid = false, ajv.errors = null

Notice in the last line Test 1 is not valid but contains no errors anymore.

The cause is fairly obvious but it is an easy mistake made given how the library currently works. Why not have var valid = ajv.validate('testSchema', data); return a result object containing errors, errorText etc. (a sort of ValidationResult object)?

Another option to avoid misuse of the function is to rename ajv.errors to ajv.lastErrors. Not that I like that name, but it makes its intended use more clear.

I understand these changes would be backwards incompatible, but it might be worth a thought if planning a big new release.

But overall; thanks for the great library! 👍