Make validate side-effect free / handle Immutable data

[smeijer]

What version of Ajv are you using?
5.2.2, latest at the moment of writing

The problem you want to solve
I don't want to modify properties in a different scope, to prevent unwanted side-effects. Side-effects are
often responsible for causing bugs.

schema:

import Ajv from 'ajv';
import deepFreeze from 'deep-freeze';

var ajv = new Ajv({ useDefaults: true });
var schema = {
  "type": "object",
  "properties": {
    "foo": { "type": "number" },
    "bar": { "type": "string", "default": "baz" }
  },
  "required": [ "foo", "bar" ]
};

var data = deepFreeze({ "foo": 1 });

issue:

var validate = ajv.compile(schema);
var isValid = validate(data); // THROW: ​​Can't add property bar, object is not extensible​​

// expected results without `deepFreeze`
console.log(validate.errors); // null
console.log(isValid); // true
console.log(data); // { foo: 1, bar: 'baz' }

What do you think is the correct solution to the problem?
The correct solution would be to make ajv behave side-effect free. Make the validate function side-effect free / immutable, don't reassign arguments there.

var validate = ajv.compile(schema);
var result = validate(data);

console.log(result.errors); // null
console.log(result.isValid); // true
console.log(result.data); // { foo: 1, bar: 'baz' }
console.log(data); // { foo: 1 }

If validate does not modify data, be sure that result.data === data. When it does modify data (because of applying defaults, or Coercing data types), result.data !== data

Will you be able to implement it
At this moment probably not, because of lack of time. Maybe in the future, but don't hold your breath.

[epoberezkin]

duplicate of #549

Please comment there.